I can help you in the MLP area, at least *G*.  
Unfortunately, I only know of cels from MLP Tales 
that are currently available.  I've never seen one 
from one of the earlier shows (besides a color 
model I have of Windy ^_^, and a few other color 
models that I saw years ago with other 
collectors):

http://www.legacy-animation.com/production_cels/ml
p/index.htm

Legacy might have some of the other shows for 
sale.

You might also want to check out Van Eaton 
Galleries.  They don't have any mlp cels, but they 
might have the other shows you're interested in:

http://vegalleries.com/

Good luck!

Talia