Ironically, Voltron just appeared (and was 
immediately bought) on Ichiban Anime's site:

http://www.ichiban-anime.com/

You could always ask them if they plan on getting 
more in.

As for the others, I've never seen cels, but I 
haven't looked specifically for those series.  I 
think mouse is right in that your best bet is to 
check out some of the big galleries.  There are a 
few online that have large selections of older 
cels.

Good luck!

JP

PS---Stupid trivia for the day: One young animator 
who started out on Thundercats later graduated to 
producing and creating his own show that some of 
you might have heard of, TENCHI MUYO!  (Mr. Hiroki 
Hayashi)  ^_^