Yup, there are a couple collectors who are there 
everyday at the Nakano Mandarake.  You have to be 
there around 9:00 to 9:30 am to beat them in 
line.  They open at 12.  But, the best way to 
find cels is to ask those collectors.  Your 
chances are so much better if you have a Japanese 
list you can ask to pass around to them.  So far, 
I have founds some nice OP cels for the Yuusha 
series I collect through a collector of a friend 
who works at Anime Shop Zero.  It's best not to 
think you are the only one who collects these 
series.  It's probably gone because either a 
collector has collected them all maybe,... or 
just burned, or sitting in a warehouse somewhere, 
or sellers might think they arent worth putting 
out since they might not sell?  Try getting the 
message out to people as to what cels you are 
collecting. I still have no idea what these 
obscure series you are referring to.

RX78GP02A