Thats because there aren't any,crazy ain't it! not even the new RD600 can provide dual x16 slots!
You'll be told by others that dual x8 slots are currently enough for crossfire but I wouldn't pay in the region of,in some cases £200! for a board that is unable to offer maximum possible performance and future proofing for my chosen graphics setup,Dual x8 maybe enough now,but for how long?
It's a poor showing for crossfire users from 775 boards at present.