Bulk buying is probably hard, but ex-enterprise Intel 10G on eBay tends to be pretty inexpensive. Dual spf+ x520 cards are regularly available for $10. Dual 10g-base-t x540 cards run a bit more, with more variance, $15-$25. No 2.5/5Gb support, but my 10g network equipment can't do those speeds either, so no big deal. These are almost all x8 cards, so you need a slot that can accomidate them, but x4 electrical should be fine (I've seen reports that some enterprise gear has trouble working properly in x1/x4 slots beyond bandwidth restrictions which shouldn't be a problem; if a dual port card needs x8 and you only have x4 and only use a single port, that should be fine)
I think all of mine can pxeboot, but sometimes you have to fiddle with the eeprom tools, and they might be legacy only, no uefi pxe, but that's fine for me.
And you usually have to be ok with running them with no brackets, cause they usually come with low profile brackets only.