How about msi big bang marshal?
This one has up to 8 pcie x16 slots and is known to work for 8 cards. Current second-hand price is about 150$ in my local market.
But I also heard someone said he could only manage it to work with at most 6 cards.
Yea, thats a good board... I'm just looking for something out of the new series that will be available in large quantities if needed and will have a longer warranty.