Only place I've seen these on is eBay, but they are REALLY expensive for a second hand part(up to £100). In some models you also need to buy a caddy (another £15) to attach them to the logic board.
Although ascetically pleasing, I don't think they are worth the money for a 11Mbps connection. You would be better off with an ethernet to wireless converter, or a Belkin USB Wi-Fi dongle if you need the portability (avoid the D-Link ones).