1- Where can you buy a lot of the supplies?
---Go to a craft/art supply store, like MJD, 
Michael's, Hobby Lobby, Garden Ridge, etc. 
Chances are, you can find a frame or pre-cut mats 
at such places, at lower prices than you would 
pay if you went to an art gallery or an art 
specialty store. 

2- What do you use to keep lines from fading?
---Heat and light and radiation are going to get 
through, no matter what. A cel that is kept out 
in the open is going to deteriorate, regardless 
of what you do. However, you can slow down that 
deterioration by using glass that has a coating 
to filter out UV light, and by not hanging it in 
a place that is constantly bright or ever gets 
direct sunlight. This is going to be especially 
true if you have a lot of warmer colors in the 
cel (reds and yellows and browns), as opposed to 
cooler colors (blues and purples), which are more 
resistant.

3- What is the best way to hold the cel in the 
frame?
---Your mileage may vary, but I like those acid-
free photo corners. You can stick them to your 
backing, and that way, the cel isn't damaged by 
anything. This might only be a problem if you're 
framing it with an original background.

4- What is the best type of boards to frame them 
with?
---Acid-free everything. ^_^ You'll probably want 
acid-free matboard in the front, and perhaps acid-
free gatorfoam or something behind it.

Ultimately, though, if you aren't comfortable 
with the job you can do on it yourself (or if you 
have difficulty gathering high-quality 
materials), you'll want to ask an art gallery to 
frame it, rather than a hobby store. Art gallery 
employees are used to handling valuable art, and 
treat it well--- and have the added bonus of 
being generally small, with a narrow clientele. 
Art gallery people tend to be very easy to work 
with and helpful, in my experience, as compared 
to a lot of the hobby-shop framers I've talked 
to. The higher cost is worth the security. 
Usually, a do-it-yourself framejob will run about 
$25 for the glass and mat and peripherals (photo 
corners, hangers, etc), plus however much your 
frame costs. So if it's a special cel, you might 
want to just go ahead and let a professional 
gallery take care of it for you; but if you're 
comfy with do-it-yourselfing, go for it! :o)