Which models of Performas? The Performa name was almost like another brand name for the Macs (kinda like GM has its divisions that sell basically the same car under different names). There were lots of different systems sold under that name.
If it has PCI slots, then you can probably use generic cards with a Realtek chipset. Last I checked, there were Mac drivers on Realteks ftp site. Also look here for a good price on a card that is listed as Mac compatible:
Computer Geeks
If you dont actually have PCI slots, you're not totally out of luck. Just about any Mac with an expansion slot can take an ethernet card, just a matter of how much you have to spend.