You can get sheets of Lexan at many home improvement centers like Lowe's, Home Depot, Menards, etc. I've heard good things about
GraphiteElegance to get the CF you need. However, it's not as "pretty" as stock pieces as there is some surface pitting between the weaves.
If you're not concerned with cosmetics as much, you could also use fiberglass. It is easy to work with and available locally. Just pick up the resin/hardener at most department store automotive departments. You can use either real fiberglass cloth or fleece (available in the craft section). I always look at the remnant section as it's cheaper. Color of the fabric isn't important IMO as you'll probably be painting it. The fleece is easier to work with IMO. Just stretch it over a frame (cover with wax paper first) and then saturate with resin/hardener. Do the otherside as well and you'll have a nice sturdy case. A little sanding and it can look really nice.