Some dealers have connections with the anime
studios or artists who work there, so they can 
get the hanken cels easier.  Some of them are
available for sale or trade in some conventions
there, the dealers often buy or trade their cels
for them.

Most cels in Yahoo Japan are craps, but there're
some rare and nice quality cels popping here and
there.  It's just liked EBay, most cels that
people put up there suck too, but rare gems can 
be found once in a while.  Most Japanese sellers
don't like to deal with foreign buyers, so it's
always best to find a friend in Japan to help you.