Hi!
Fortunately I've not had this problem yet myself, although over the last few weeks in my flare I was frightened of it happening and wore extra underwear to bed just in case.
My children have an extra 'sheet pad' on their beds in case of occassional accidents. It's a pad about 3ft X 3ft 6in, with an absorbent surface and waterproof backing. You can put it either under or on top of the sheet, and it's machine-washable. (We also had a waterproof cover immediately on top of the mattress to protect that, too.) What I used to do when I had to change sheets a lot was to put a sheet on the bed, then the pad and another sheet on top. When an accident happened in the middle of the night you can then just whip off the top sheet and pad, and you have a clean sheet already in place.
You can usually get these products in most bedding departments or children's shops or pharmacies.