I've found all my used kart/parts bargains on craigslist. Being you mentioned Birel, I assume you want to buy a race kart. Instead of just buying a bare frame I'd sugest you look for a complete used rolling chassis. (less engine)
If you get on craigslist, preform searches for each of the terms/spellings
go kart, gokart, go cart, gocart cause everyone seems to spell the term differently. Also I cant tell were your located but unless your in a big huge town, you should try searching all your surrounding towns/whole-state craigslists cause theres lots of smokin used-race kart deals out there right now.
Here's a few sources for Birel parts:
http://www.dpkmotorsports.com/shop/
http://www.mrpmotorsport.com/
http://www.gobirel.com/
I couldnt find any big online Birel parts catalogs but most any of those places should have parts avalible if you contact them.