Where can I get single water proof sheets?

Either water proof undersheets or traditional waterproof rubber sheets.

If you are having problems with bed wetting we went to Home Pro and just got some vinyl sheet (sold per metre off the roll) for our daughter when she was young.

Is this what you want it for?

Yes.

We are currenlty using plastic bin bags, but the vinyl sheet would work.

She is a bit unsettled with the school holidays and not doing so well this week.

I found some waterproof bed sheet at Tesco 2 years ago. I do not know if they still have them.

Central - where are you located?? Also larger pharmacies might be worth a try?