Hmmm, that's easier said than done.

Zetsuai/Bronze cels are nigh impossible, it 
seems. I've only ever seen one, and that was in a 
collector's gallery. Never for sale. Expect a 
fairly steep price if you do find them. 
Personally, I'd love to find an Izumi cel, but it 
seems a vain hope about now.

Ai no Kusabi cels are not quite as hard to find, 
but still extremely scarce. I think there are 
only a handful of collectors that frequent these 
boards that have them. (Unfortunately I'm not one 
of them. ::sigh::) They do turn up on Yahoo Japan 
every now and then, but again, expect steep 
prices and fierce competition.

There are other yaoi series that have recently 
become much easier to find thanks to Anime Taro. 
You might run over there and check them out. =)

--Jennifer