Ikea and Carrefour have good ones. Try to get the fitted sheet type with the soft rubbery backing - they are more comfortable to sleep on. The other plastic type ones get very hot, move aroung the bed and also make a crackly noise.
Boots have disposables sheets you can put under or on top of your sheet, the have an absorbent side with waterproof backing. That would absorb any sudden water leaks and give you peace of mind. They come in packets in the baby and mother items area.
Have seen king size matress protectors in Geant too.
<em>edited by Di@DXB on 21/07/2011</em>
I know it sounds like an odd request, but am 35 weeks pregnant and starting to freak out about waters breaking all over the bed!! My nesting instinct means that i cannot rest until i find a suitable one. I hope someone out there can help. :) Thanks all.