I wouldn't bother with any fancy media converter, you can buy a beat-up old hub with a BNC connector and RJ45 connectors off of eBay to get the old card on the network.
Just make sure you pick up a pair of 10Base2 splitters (Ts) and terminators along with the cable, some more eBay items. Bonus, you've just learned how to implement a very basic 10Base2 network (not that it'll come in handy ever, most likely.)
I put together the same setup for my 4000T with Hydra card for roughly $20, not counting the cable I impatiently bought at Radio Shack (another $10 or so.)
Of course, then you've only got one machine networked. You'd need additional cards for the others.
