I've succesfully tested two options:

1. As suggested by Kronos - PCI WiFi card with Atheros 500x chipset. I've succesfully tested TP-Link TL-WN 350G card in Power Mac G4 and Pegasos II. Costs less than 10 EUR.

2. WiFi minirouter in client mode, connecting to your home WiFi network. I've succesfully tested TP-Link TL-WR 702N and use it currently with Mac mini (my mini has no AirPort module). Costs around 20 EUR.