Looking quickly at the US Ebay just now, here are some older single port ConnectX-2 cards:
https://www.ebay.com/itm/Mellanox-MHQH19B-XTR-PCI-E-Single-P...
US $23.00 each, free shipping. Note - I don't know the seller at all, this is just from a quick look on Ebay.
There's a tonne of stuff on there. ConnectX-2 is no longer supported by Mellanox, though the cards themselves are generally pretty bullet proof.
Drivers for Linux and FreeBSD come with the OS. :)
Drivers for Windows are a bit more complicated. Mellanox has an archive section with the ConnectX-2 drivers, which work for most people.
Cabling directly between two cards - instead of going via a switch - is pretty common for people just trying out the tech. It lets them plug one card into (say) a FreeNAS server, with the other card in their desktop or workstation. Removes the ~120MB/s limit of 1GbE, assuming any kind of reasonable disks in the connected NAS. :)
If this kind of thing is of interest, probably the best place on the whole internet (not joking) for this stuff is Serve The Homes' Networking forum:
https://forums.servethehome.com/index.php?forums/networking....
Plenty of advice and stuff to read there, and a good place to ask questions from people that have a clue. :)
That's for storage physically located nearby though. eg same building, no more than 1 or two network hops (roughly).
Higher end networking from the last decade or so has become really fast. :)