When I was a bedwetter my mom used to put three or four waterproof pads (plastic on one side) under my sheet covering the area where the wetting was likley to occur. We also put one fitted absorbant matress cover over it which is what I guess you are having trouble finding. At a motel I recently stayed at they had a non fitted sheet that looked like it would work pretty well. Maybe if you can buy one of those for a twin sized bed and put smaller waterproof pads under/around it it would work as you wouldn't likley wet where your pillow or feet would be.
I know the stuff I had was washable and sturdy because it got it got sent through the washing machine almost every other day from when I was about 10 to when I was 16. When/if the sheets get wet you can always strip the bed and throw it all in the washer and the matress should be fine. It also should stay in its place on the bed aslong as you are careful when making it. All the stuff I had could fit into one load of wash making it fairly discreet.
These are similar to the pads I was talking about
:
http://www.amazon.com/Waterproof-Cot.../dp/B0007YJIIO
This, while not what I had, also seems to be the most common product from google search:
http://www.angelfacesafety.com/bed-w...full-size.html
If you get a couple pads you can use them in strategic locations under something absorbant like that.