Look for the old lucent orinoco gold/silver cards. You can probably find one used for not too much. Note that the newer proxim orinoco cards don't external antenna jacks. By default these cards only supported WEP, but you can find updated drivers with WPA support by following the link on this page: http://www.netstumbler.org/showthread.php?t=8885
I've been using one for about 5 years, and it's worked great so far.
Edit:
It also might be worthwhile to post a WTB thread in the For Sale/Trade forum. Just say you want a card with an antenna port, and I'm sure someone has an old card they aren't using.