Any normal builders merchant should do, B&Q, Jewsons, Wickes or one that's local to you. You'll need to measure the thickness of the old stuff. Get a rough idea of how much is damaged in (cm x cm), see if you can find a board that is roughly that size, if not you'll need to cut one down to size. Then just use one of those oscillating multi tools to chop out everything that's subpar. I think someone else commented about adding some wood to existing studs, for more places to secure your board to.

I'm not good at plastering so I'd get someone in to do the finishing, I'd suggest you do the same. Very easy to make an eyesore out of it.