I know that this is an ancient thread, but I just waded through it because I'm upgrading my pfSense box and wanted to move from an i340-t4 to an i350-t4 due to SR-IOV support and have another option that I didn't see mentioned.
In the distant past I put together a few low-cost packet generator systems for testing networking equipment and back then Silicom adapters were one of the cheapest ways to get multi-port ethernet cards (especially the `bypass' adapters, I think because they look weird and freak people out). If someone wants a cheap `most likely not fake' i350-AM4 card, these are currently cheaper than other options I've seen, generally between $20 and $30 each on eBay:
Silicom PE2G4BPI35LA (low profile quad port i350-AM4 bypass adapter)
Silicom PE2G4BPI35A (full height quad port i350-AM4 bypass adapter)
Silicom's Quad port 1Gigabit Copper Ethernet PCI Express Bypass Card Intel® i350AM4 Based supports Normal, Disconnect and Bypass modes. In Normal mode, the
www.silicom-usa.com
Silicom PE2G4I35L (low profile quad port i350-AM4)
Silicom's Quad Port Copper 1G Ethernet PCI Express Network Interface Card Intel® i350AM4 Based is PCI-Express X4 Copper Gigabit Ethernet network interface
www.silicom-usa.com
Silicom PE2G4I35 (full height quad port i350-AM4)
Silicom Quad Port Copper Gigabit Network Ethernet Cloud Computing Card Intel® i350AM4 Based is PCI-Express X4 Copper Gigabit Ethernet network
www.silicom-usa.com
I haven't used any of these newer cards myself yet, but with the old bypass adapters if they were set to `bypass mode' (electrically connecting each adjacent ethernet port together using relays on the card, to bypass the card completely) you just needed to install the bypass driver to set it to normal mode and the card would remember it, then from there on out it could be treated just like a normal intel ethernet card. Interestingly, the datasheet for the bypass version of the low profile card claims that it only uses 4.68W in 1G normal mode, whereas the regular card uses 5.04W. I'm not sure just how that is the case, but for $19 I figure I'll check it out!
They also have different versions with either SFP cages or built-in optics, but I imagine that if you were going to do that you'd probably just get a cheap 10G card!