apart from obviously eBay,
http://www.amibay.co.uk/ is a good place. Most of the sellers and buyers are from the amiga.org and amiworld portals and it seems a quite pleasant/hassle-free little community based trading post. You should be able to pick-up an 030 quite cheaply, they're among the most common accessories.
But I'd suggest you get an 060 if you can.