I would have to say definitely go with 
www.vegalleries.com.  I buy a TON of stuff from 
them.  They are only about a 20 minute drive from 
my college so I pop in a lot.  They do have a 
huge selection of their stuff online- but since I 
go into the actual gallery quite often, there are 
tons of items they haven't scanned yet for their 
website.  So if you have any particular cartoons 
you are looking for- drop them a note- they might 
have it and they will be happy to scan it for you 
to show you.

If you need any more ideas for galleries that 
specialize in Western Animation, drop me a line 
and I'll help you however I can :)