You probably have these already, but I guess 
it's worth a shot. ^^

He-man and She-ra:
http://originalcartooncels.com/
(should come with a certificate of authenticy)

More He-man and She-ra:
http://www.vegalleries.com/filmaopc3.html

He-man on ebay: http://search.ebay.com/he-man-
cel_W0QQsofocusZbsQQsbrftogZ1QQfromZR10QQsatitleZ
he-manQ20celQQsacatZ-1QQcatrefZC6QQsargnZ-
1QQsaslcZ2QQsadisZ200QQfposZQ5AIPQ2FPostalQQftrtZ
1QQftrvZ1QQfsopZ1QQfsooZ1

I can't say that I have comed across any cels 
regarding thundercats, but I'll be on the look 
out and let you know if I see anything.

Best of luck with your search! :)