I saw some Mysterious City of Gold cels at the 
Anime Museum: 
http://www.animemuseum.biz/index_plist.php?
lan=en&show=cels&title=0822&p=1
(I haven't ordered anything from there my self, 
but the site looks serious enough. ^^)

As for Captain Future I haven't comed across 
anything, it wont happen to have a different 
name perhaps?
I found Future Boy Conan if that's the same 
show: 
http://www.kakuto.com/ms2fr/catalog/index.php?
cPath=23_143_104 (I have ordered there and found 
it a good and fast store by the way).
There are also a lot of Captain Harlock cels in 
several shops, but I presume it's not the anime 
you're looking for.


Sorry if I wasn't of much help, I'll try to 
search for more tomorrow. By the way, yhile 
you're searching for cels from older animes, 
could you please tell me if you should happen to 
stumble upon anything from D'Artagnan and the 
Three Musketeers? ^_^

Best of luck with your collection! :)