Where can I get a good plastic protective cover for a double bed?

[Gateau]

DS(2) was sick in is bed the other night and a bit of it seeped through to the mattress. Not a lot, thank goodness, but it alerted me to the fact that we need to invest in a plastic cover, not least to cope with bed-wetting in the future!
DS sleeps in a double bed; does yone know where we can get a good cover at a reasonable price?

I got one in dunhelm mill last year, think it was about £15.

Thank you, bobs.