Try
www.ukcampsites.co.uk - there are some nice sites around but it's been such a long time that I can't remember their names.
I've stayed in a decent one (Camping and Caravanning Club near Yarmouth) and one near Sandown. There's a nice one near Brightstone Bay - I've not stayed there, but ridden above it. I think it's called Grange Farm.
Sorry that's a bit vague.