Hi Anna. 

I found my two Voltron cels on YJ, completely by 
chance. They were marked as "unknown" - I've 
seen that sometimes with older series. Anime 
Gambit has a few cels from Voltes V. The easiest 
older series I have had luck with has been 
Spaceketeers (SF Saiyuki Starzinger) from the 
Force Five block of anime that came stateside 
(it did come over here, right?) Those you can 
mainly find on YJ; also anime-cel.com has a few. 

I have noticed that the competetion has been 
growing for some of these older shows, but you 
can still find lots of good, inexpensive stuff 
out there. Good luck! ^_^