The Mellanox ConnectX-2 are relatively cheap, and they seem to be everywhere. However they are not compatible with PFSense because Mellanox stopped supporting the older cards on BSD. Though they still work on FreeNAS.
Chelsio cards are higher up on the list for universal compatibility. I have one on the way to put in the Firewall.
A quick search on Ebay can get you one for relatively inexpensive.