There isn't one specific site the sells ALL of their markers for cheap. You just have to shop around for the best deals.
If your looking for a new or refurbished (practically new!) gun, then check out the auction pages on www.actionvillage.com
If you want to find a gun for cheaper, then second hand ones on auctions are one option. Check www.ebay.com for one auction site. www.yahoo.com does them too.
You can check the classifieds on this site (although most will be in england) and the classifieds from other paintball forums on the net.
The only thing with buying second hand - especially on auction is that you don't always know what your going to be getting. It could be a piece of crap if you're not lucky. - On the other hand you can get some remarkable deals.
Another option is to e-mail and ring of paintball shops to see if they have any ex-display or demonstrations models for sale. You can usually get a bargain that way. A mate of mine got a Dark Angel LCD for $500 as an ex-demo -only a couple of thousands balls through the baby!!