If you're interested in making a copy of your 
cels for framing/display purposes, and you want 
to leave the original safely in its binder, a 
cheaper, less time-consuming method might be to 
make a high-quality color copy of the cel and 
frame that. I think that the concept of lazer 
backgrounds are similar to this concept--- 
there's only one background for a cut, but in 
some cases, people will make color copies and 
distribute them with the cels when the sequence 
is broken up. You don't have the "original" 
background, but just looking at it, you can't 
tell--- and it gives the cel a feeling of 
completeness.

I think if you were planning on providing this 
service to other people, it would be very 
difficult to persuade them to send you their 
actual cels, especially since it's the rarer, 
more high-level images that are probably going to 
be in question. Even if you had a good, 
established reputation, it would probably be a 
bit difficult to even get them to send the 
sketches, if they had them in their possession. 
^_~ You could probably work well from a scan they 
provided, though differences in monitors may give 
some difficulties, especially when it comes to 
color-matching.

Studios do not mix their own paint. They buy 
paint pre-mixed from the manufacturer. (And I 
believe they use custom colors?) If you find an 
animation supply company, animation paint will be 
available, but what you may buy by the tube or 
bottle may be different from what they may buy by 
the bucket. :o)

Good luck!
-Cres