I've been looking for 6990s and even 5850s on [H] recently and no luck. One guy was selling a used 6950 that he claimed was "clocked exactly the same as a 6970" (which might be true for gaming, but does it have the same number of stream processors as a 6970?) for a fairly decent price. I considered buying it until I looked at the hardware comparison wiki and the $/MH/s ratio seemed very poor.
But just saying, I haven't seen 6990s anywhere
because I would've grabbed at least one, and I'm currently waiting on a couple back-ordered 5850s as they're a good, reduced price and have an excellent $/MH/s ratio, IMO. But they're also nowhere to be found. A couple remain in stock for $189.99 on Newegg but that's hardly a good buy for 330MH/s.
I laughed a bit when people started talking about the supply of AMD video cards drying up ... until I tried to buy more. Seems like stocks of the 5-series cards are all but dried up (5770s are still available for a good price, $115, but those aren't gonna last forever) and even the high-end 6-series cards are being snapped up as soon as they're off the line as near as I can tell.
This is almost a blessing in disguise for me, though. Waiting on a couple parts for my third miner and then I have to take a break for awhile.