I use a 3Com Etherlink III LAN PC Card with my A1200 (which I'm posting with now). It's well supported and seems pretty popular with other A1200 owners I've spoken with. If you search eBay for Etherlink, find one of these 3Com Etherlink IIIs and you can't go wrong. It works with AmiTCP and Miami using the 3Com driver on Aminet -
http://aminet.net/package/driver/net/3c589For a list of compatible and incompatible PCMCIA network cards for the Amiga, check this site. If you have anything to add to the list, you can mail GMB and he'll update the list.
http://www.g-mb.de/pcmcia_e.html