The bulk of my recent Weiss cels have come from 
Japan directly (YJ and a direct buy through 
Celga, including Nagi, an Aya OP and a cel of Ran 
smiling ^_^). Celga quite likely still has some 
reasonably priced OP cels of Aya and/or Yohji if 
you contact them directly(bustakei@yahoo.com).

Animegame and Usagi's House both have some Weiss 
cels right now. And Raxa often posts Weiss cels 
in his Ebay auctions. In fact, there's at least 
one cel of Aya up right now, earring visible, 
though I don't recall whose auction it is.

The truly hard part is landing a cel that's of 
good quality, as the animation in the show varies 
wildly. And be prepared to pay a LOT for a good 
cel of Nagi, as he's terribly elusive.

A.