X-Surf is the first one that came to my mind as well since they do seem to still be available, but I wouldn't pay either X-Surf or Deneb prices unless I really needed to get online. I'd stake out eBay for the odd C= A2065 or Hydra Ethernet boards, they can be had in the $50-$70 range occasionally. They're oldschool though, you'll need a cheap media converter (usually AUI to RJ45, but there are some BNC only models out there, those would be my last resort.) Ariadne is another original card, but they seem to go for more when they come up.
