Well, having surfed other sites for this stuff I can tell
you that what we would consider "cel bags" may also be called
envelopes, sheet protectors, comic/magazine covers, 
photo sleeves, etc.etc.etc.

The most important thing is you want to know what dimensions
they're selling - while comic bags are extremely common and
easy to get, the problem is that they're not commonly sold in
the sizes we need.

I ordered the "Mylar Envelopes" at the Talas site, 11"x17"
which fits normal cels and oversized/backgrounded cels quite
nicely. 

I do NOT recommend getting anything thicker than 3mil, though!
I didn't realize the Mylars were so stiff (even if I like it
that way, my celbook is pretty damn full now), otherwise I'd
get 2-2.5mil. Oh well. Another lesson learned.