In the old days, there are various channels for 
the dealers purchasing their cels, here're some 
examples.

#1 - connection with the studios, some dealers 
have good relationships or even partnerships with 
them, ex: Anime Link and Madhouse.  Or another 
case is Anime World Star buys so many boxes of 
cels from the studios directly, some cels in 
these boxes are good, some are junks.

#2 - Toei and AIC used to have cel events, so 
fans and dealers bought the cels there.  There're 
also some other events that Japanese dealers or 
collectors bring their cels there for trade and 
sale.

#3 - Studio often gave out their cels as gifts to 
various promotional events, like movie premiere, 
magazine promotions..etc.  Then the winners often 
re-sell these cels to the dealers.  

#4 - The people who work inside the studios 
sometimes take (steal) the cels to the market for 
sale.

Most of the time, the foreign cel dealers have 
connections with Japanese dealers, they act like 
middle-man and often get the good cels to the US 
or European cel dealers.