I agree with what has been said. Though if you're really in a bind, you could get washable water-proof plastic/paper/whatever that you stick under sheets, and then go to a fabric store and pick out the softest fabric of your choice for two sides, cut everything to what you want, sew it, and presto!
In fact, the paper might not be good or too thin, so why not try a cheap shower curtain? (The plastic ones, not the fabric ones.) You can wash them in the washer just fine, cut them to fit (buy the biiiiig ones just to be sure) and then get the fabric you want. :3 just do put in the dryer, or if you do, do low knits/delicates/air dry. Or hang it.
...now I have the idea to make my own water-proof throes to sell! XD I loooove the minky fabric, I might have to do it now and see how well it goes!
Just remember to sew the edges with the curtain sewn in with it, or it'll bunch up inside. I also suggest adding an edging all around, too.
Anyone else done this before and have it work?