You have, in fact, found a website with cheap affordable bikes. See that tab at the top of this page that says "Classifieds"? Click that, then click "Full Suspension" and browse or search. I've seen plenty of good deals in there.
Pinkbike.com's classifieds get a TON of traffic, but a lot of those are from Canada and I think people have had some sketchy experiences with sellers from there.
Another place to check is the classifieds on mtbr.com, but there don't seem to be as many DH bikes posted there.
If you're feeling brave, you can check out ebay, but I would stick to local buyers who'll let you take a look at the bike before you buy. That's a good rule of thumb for any used bike, actually. I bought my DH bike used on ebay and got an amazing deal.
I don't know what your definition of cheap is but I do have a mint Downhill bike for sale.
More pics upon request.
Frame: Easton 7005 FS RAD DH Taperwall Aluminum
Frame Weight: 5.9 kg / 13 lb (18.0")
Fork: Marzocchi 888RC2X
Front Travel: 200mm
Rear Shock: FOX DHX 5.0
Rear Travel: 203mm
Head Set: FSA Pig DH Pro
Stem: Marzocchi Clamp - 31.8mm
Handlebar: Easton EA70 - 31.8mm
Brakes: Shimano Saint
Brake Levers: Shimano Saint
Shifters: Shimano XT Rapid Fire
Gearing (RR/FR): Shimano Saint
Cranks & Chainrings: Shimano Saint double
Bottom Bracket: Shimano Saint M800
Pedals: Shimano MX30
Hubs (RR/FR): Shimano Saint 150mm /20mm
Cog-Set: Shimano XT
Chain: Shimano HG73-9
Spokes: DT Champion
Rims: Mavic EX 729 Disc
Tires (RR/FR): Kenda Nevegal 2.5" Tires
Seat Post: Easton EA70
Saddle: SDG Freeride
Frame & swingarm were always protected by 3M like covering. Frame/swingarm is in immaculate condition, no cable rub, scratches, or rock chips. Only the bottom of the cranks have minor scratches on them, which you can see in the pictures above. Bicycle is completely stock, minimal use. $2500.