As
SirDice pointed out, there are only 2.5GBit NICs that actually were long dead - or better say: never lived because 10G was already there and cheap, but now they seem to reappear on some desktop/gaming gear. I actually didn't know there were even 2.5GBit PCIe NICs available; I only ever saw those as onboard NICs and I would never ever buy one of those dead births on purpose - go for 10GBit, this is what everybody agreed on as the next step up from 1GBit and hardware is widely available, cheap and well supported. In fact 2.5/5GBit actually only is a crippled variation of the 10GBASE-T standard to make it work on really old cabling (cat5) that should have been replaced 10 years ago...
If you absolutely have to use a 2.5G NIC (and throw it away in 2-3 years because it is again obsolete...) there is ongoing work to support the various Intel I229 variants, which are already working in the development branch (CURRENT) and IIRC patches for 13.0-STABLE are also already there. If you search for that chipset on the forum there should be several threads about the current state and how to apply the patches.
OR you can get e.g. one of the later Intel 500 series NICs (e.g. X550) which are 10GBit but also support lower speeds (5/2.5/1Gbit). These are supported by the ixgbe driver and the 500 chipsets have been around for quite a while and proven to be reliable workhorses.