No, you are confusing with the sales that AIC
was doing 1 or 2 time a year. But they stoped
doing those sale a couple of years ago.
(You didn't had to pay an entrance fee but you
had to register in advance. Then people were
going by group of 20-30. Each person had a stack
of cels and you had 10 mins to select those you
wanted. Then you had 10 mins to pick cels among
those discarded by others).

Dealers get their cels from shop, from collectors
and from various small "events". (For example,
you may get a cel when buying a DVD box of the
anime. Or when going to the theater to watch it).
Also, studio are sometime selling some cels but
usualy it's for advertising so you are limited
to a very few cels per person.
Japanese dealers usualy ask many other person to
go with them to those events to get more cels.

And naturaly, it's sometime possible to get large
quantity of cels directly from studio. But that's
very difficult even for japanese dealers/shop.

Cheers,
Yann Stettler