There are a number of wonderful online fabric shops that stock organic fabrics at competitive prices. Here are a few to start with, and see if you have a local store that stocks organic goods:
Near Sea Naturals stocks brushed organic percale, 113.5 inches wide, and organic sateen, 112 inches wide, both in a variety of low-impact dyed colors and prints.
Janice's stocks organic cotton sheeting in neutral, 96 inches wide.
Organic Cotton Plus: stocks organic sateen in a variety of colors and prints, 113 inches wide, and organic cotton flannel in varying neutral tones, 84 inches wide.
And don't forget that you can still visit thrift stores to find oversize sheets that you can cut down to your specific needs.
Measuring, Ordering, + Preshrinking
Wide-width fabrics, such as the ones I’ve noted above, are the best choice for making your own sheets, as they will reduce the number of long seams you will need to sew. The easiest thing to do is measure a set of sheets you already have that fit the way you want them to.
Or, measure your mattress and pillows themselves to help you figure out how large your final items should be. Measuring for your fitted sheet will be the hardest. First, determine the height, width, and length of your mattress. Then, to each of those measurements add 2 inches for hems on all four sides. So, for instance, if you have a full-size bed that is 54 inches wide, 75 inches long and 10 inches deep, you'll need a piece of fabric that is 78 inches wide and 99 inches long—basically adding 12 inches (the 10-inch depth plus 2 inches for hems) to all four sides.
Flat sheets and pillowcases are easier to fit. If the sides of your sheets tend to pull up and let in drafts during the night, or if you have a very thick mattress, you may want to add a little extra width to flat sheets: I like to have enough to tuck in about 4 inches along each side and 8 inches at the foot end to hold the sheet firmly. Once you know how big you want your sheet to be, add 3 inches to the width and 5 inches to the length for hems along the sides and bottom.
A standard-size pillowcase is 20 by 26 inches, and king-size pillowcases are 20 by 36 inches. As for fabric needed, double the length, and add 6 inches for the hem around the opening and add 1 inch to either for the side seams. Your final fabric need would be 22 inches wide by 58 inches long. If you have smaller pillows that need custom cases, measure the diameter of the pillow and divide that in half to get the width, measure the length of the pillow, and add the same 2 inches to the sides and 6 inches to the length for hems.
Once you've calculated how many yards of fabric you will need (add up all your length measurements and divide by 36 inches, keeping in mind that most fabrics come in predetermined widths), add a little extra for preshrinking. Unless you are completely sure the fabric you are buying is really and truly preshrunk (most fabric isn’t), you will need to order extra so you can preshrink it yourself. This is very important, especially with fitted sheets! It is a good idea to add at least 10 percent to your raw yardage to allow for preshrinking, but ask your supplier if that will be enough. You may have a little extra, but that's easier to deal with than winding up a few inches short. To preshrink it, simply wash and dry your fabric in the hottest water and in hottest drying cycle you have before you make a single cut. (If you are cutting down pre-owned sheets, this step will have already been taken care of for you.)
This is the easy part! In addition to your fabric, you need a large spool of matching thread, and 1 yard of ¼-inch (or slightly wider) elastic for your fitted sheet.
To sew a flat sheet: Sew the wide hem that will be the top of your sheet first. Fold the top edge over ½ inch and iron the fold flat. Measure and fold that over another 4½ inches and iron that fold flat, too (the folded-under rough edge will now be hidden). Pin everything down, and sew through all three layers of fabric about ¼ inch from the bottom. Then, sew through the two layers of fabric at the top of your sheet about ¼ inch from the edge. Finally, sew narrow hems, about 1½ inches each, along the sides and the bottom edge of your sheet. Done!
To sew a fitted sheet: Throw your fitted-sheet fabric over your mattress, making sure it's centered (if you use a mattress cover or pad, be sure that is on the mattress). Picking up one corner at a time, hold the fabric out level with the top surface of the mattress so you get a "fabric triangle;" pin the triangle of fabric together along the corner of the mattress to mark your corner seams (be careful not to pin it to the mattress at the same time). The fit should be snug but not pulled tight. Once all four corners are pinned, lift the fabric off the mattress carefully and sew seams to replace the pins. Reverse your machine for an inch or so at each end of the seam to help prevent it from splitting during use. Before you cut off any fabric, test the fit on the bed. Once you're satisfied, cut off the extra fabric on each corner, leaving ½ inch along the seam, and sew a narrow hem all the way around the bottom of the sheet.
Next, cut your elastic into four equal lengths. Center each piece of elastic at the bottom of each corner on the inside of the fabric, and pin the center to the sheet’s narrow hem. Stretch each end of the elastic as far as it will go along the edge of the sheet and pin the end to the sheet’s hem there, too. (When you let go, the sheet will bunch up; this is correct). Use a zigzag stitch to attach the elastic to the edges of the sheet: Start sewing along the edge of the sheet’s hem an inch or so beyond the elastic, use both hands to pull the sheet edge flat under the stretching elastic, and sew through the elastic and the sheet. Once you let go the elastic will contract neatly, puckering the elastic and making the fitted corner. Repeat for each corner.
Fold the cut fabric in half lengthwise to make a pillowcase shape. Sew ½-inch seams along the long sides, leaving the top unsewn. Fold ½ inch of the unsewn edge toward you and iron the fold flat. Fold that over another 2½ inches and iron that fold flat, too. Pin the edge down, and sew through all three layers of fabric about ¼ inch from the bottom fold, and then sew through the two layers at the top of your pillowcase about ¼ inch from the fold. Turn it outside in, and voilà! You've got a pillowcase.
How much can you save? Well, the raw fabric for the sheet set above cost me about $100 including shipping; thread and elastic added about $8 more. That's comparable to the low-end premade sheet sets sold online. And the fabric is made in the U.S from certified-organic cotton, something few of the inexpensive sets I saw online offered any specifics on. Plus the sheets fit, are made well, and I like the colors better than anything I saw in stores.