You can get pretty cheap 10GBASE-T NICs on ebay. I've had pretty good success with this abomination, a server-pull NIC with an HP proprietary physical interface plugged into an adapter to PCI-E: https://www.ebay.com/itm/144151881516

That's a pretty weird contraption. If you want weird, I'd suggest one of these https://www.ebay.com/itm/166884585625

Silicom PEG210 Silicom PE210G2BPI40-T-SD-BC7 Intel x540 based bypass NIC. In case you want to have the two ports connect together when the computer is off or something. Setup time is a bit more, but you can also configure them to act like normal NICs.

Usually show up around $15-25 like other x540 dual rj45 cards, but sometimes a bit less, cause they're weird.