Usually at least one cel from Lesson XX floats 
around. Taro occasionally has (and ssould still, 
don't they?) Level C cels, and Boku no Sexual 
Harassment pops up as well, as said before ^_^ 
FAKE would probably be the easiest to find out of 
all of them, as Ms. Poe said, but it's still a 
hard one to nab (I only have one cel from it yet, 
an absolutely kawaii Dee). Kizuna also has one or 
two cels circulating around, but Kizuna was OK to 
me (not my favorite yaoi series, but hell, it's 
yaoi, ne?). I've NEVER seen cels for are Kaze to 
Ki no Uta ("Song of the Wind and Trees"...quite 
probably the most influental shounen ai/yaoi work 
ever, not to mention one of the first...manga was 
by Keiko Takemiya, the same woman responsible 
for "Chikyuu e..." (Toward the Terra)), and I 
don't think I've ever seen cels for Fish in the 
Trap (although, that WAS a HORRIBLE 
anime...literally had no beginning and no 
end...the manga is much better), and never seen 
cels for any of the Bronze series floating around 
either. 

So...umm...yeah

Just keep trying...

-Misha-chan