Would just add that if you are using relatively thin media, for instance thin RC papers, you should avoid black gator because it will drag down the bright areas of the print a bit because the media is slightly translucent. But for thick, perfectly opaque media like canvas mounted with glue, black is a good choice because it visually helps you get the glue spread out evenly.
As Justan mentioned the bare edges of Gator are beyond delicate. A trivial bump into anything will dent it, and if you hand such a piece to an average buyer he will within seconds bump it against something, guaranteed. The cheap fix is to carefully trim off 1/8" or so.
Companies that supply sign makers usually carry Gator, also many suppliers of sheet plastic. I like Piedmont Plastics. Online companies like this
will sell you Gator for about 2.5 times the price of a wholesale supplier, not even counting the considerable shipping costs your local supplier doesn't charge you for. Most suppliers will cut 4x8 foot down to custom sizes for you. There is usually a charge of $10 per session for any reasonable number of cuts.
Oh, here's an online company
that sells Mighty Core in 30 x 40, 25 sheets to a carton. They're only charging about twice the wholesale price, not counting shipping. Mighty Core is a Gator clone. Comes in 1/4" instead of 3/16". I actually prefer 3/16" because of framing issues, but 1/4" would of course be more rigid and beefier looking when seen bare.