Don't know about websites, but you can definately get them in most camping shops (Blacks, Oswald Baileys etc etc) There are a few different types, recently I have some which are like a corkscrew design.
After going to Sandy Balls in the new forest I know what you mean. We use some heavy duty ones, some 10" and about 1/2" in diam (think the shop at Sandy Balls sells them funnily enough) and also some that are more like 8" nails with a plastic top. I do take a proper club hammer too, in this sort of terrain the rubber mallets aren't that useful.
We got some at the campsite shop (we forgot ALL the pegs for the tent!). We paid about 70p or each rock peg. I didn't even know they existed before so I'm rather glad we forgot the pegs afterall!
you pay about 70 p each for rock pegs. they look like large nails, usually with a plastic collar at the top, (ours are glow in the dark!)
------------- "He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion." In memory of Bear. 06/03/06 - 25/01/12 Izzie 23/09/07 - 25/03/13