Saw 8 on ebay of: Mellanox Dual Port 56gbps Infiniband plus 40GbE cards, part number MCX354A-FCBT on sale for $370 each. These are ConnectX-3 cards that will do both speeds and auto-negotiate down.
Linky: Mellanox MCX354A FCBT CX354A FDR QDR Infiniband 40 GB s Ethernet Connectx 3 2P | eBay
Linky: Mellanox MCX354A FCBT CX354A FDR QDR Infiniband 40 GB s Ethernet Connectx 3 2P | eBay