I don't get it. eBay is literally filled with them. If you search for "hp 764736" or "544+", all of these are CX3 Pro dual port 40gb cards, and just need a flash to stock Mellanox firmware. Typically less than $50.
They will do RoCE v2 all day long without breaking a sweat.
They will do RoCE v2 all day long without breaking a sweat.