If you want to add 10G to existing systems, cards with the old Niantic 82599EN chipset are cheap and widely available (now branded Intel). They use the IXGBE driver in Linux, and work out of the box on Windows as well. SFP 10G fiber transceivers are much cheaper than 10G copper: I saved money by using OM3 instead of cat6, but YMMV.
https://www.amazon.com/dp/B01LZRSQM9
https://www.amazon.com/dp/B01DCZCA3O
An old PC I built in 2013 is able to saturate one of those with room to spare.