Alrighty^^!  Perfect question for me ;)!  
Well...If you're looking for yaoi cels from the 
following series you will most likely have no 
luck in finding them for sale or auction 
anywhere: FAKE OVA, & the Kizuna OVA's.  You're 
more likely to find Kizuna cels than FAKE cels.  
I bought the only FAKE cels that were for 
sale/auction a couple years back.  I sold a 
couple from the sets they came in, & since I won 
those auctions I haven't been able to find ANY 
ANYWHERE =(!  Now, Kizuna OVA cels are rare, but 
they pop up once or twice a year...ONLY on 
auctions so far.  I have 3 in my gallery right 
now =).  Now...For the most WANTED yaoi cels...Ai 
no Kusabi....*drool*...All I can say is good 
luck...They pop-up on auctions every now & then, 
& when they do...every drooling fangirl is out to 
gobble them up -_-...The AnK prices used to be 
wonderful...now...on recent auctions...since 
everyone wants a cel...Prices have 
SKYROCKETED...A set I was bidding on with someone 
else skyrocketed to over 60000 yen -_-....So 
expect to see all future AnK prices to take a 
steep rise....Hopefully you took out that loan 
from your bank...lol....I was lucky enough to get 
a *BEAUTIFUL* cel of Riki(Thanx Sig ;D) for a 
decent price before the prices skyrocketed -_-...
Now....I know those are the cels almost all of us 
are after....There are other yaoi series like 
Level C that pop up on Taro occasionaly, & My 
Sexual Harrassment over at Anime-World-
Star...Those are fairly common yaoi series cels.