You can actually fabricate a nice canopy out of an old plastic soda pop bottle, some scrap wood, and a heat gun.

1. Step One: find pop bottle of the appropriate color and size (2 liter). I like "Sprite Zero" for the really nice, clear bottle.
2. Step Two: drink soda. This may take some time, and you should be really careful with this step. If there are females present, you may harm your chances of reproductive success with that candidate by consuming the entire bottle in front of them. And you can feel some gastrointestinal discomfort from consuming the pop. I'd suggest drinking it while you perform Step 3.
3. Step Three: whittle a suitable (male) canopy-shape out of scrap wood. If someone vacuum-formed a new canopy for you, you'd still have to make a form. Keep comparing it to your existing canopy or try it out on your plane until you get it whittle down right. The beauty of the eventual piece is directly related to how well you whittle, sand, and shape this mold.
4. Step Four: Use something to make your mold a little slippery so that the hot plastic won't stick. A coat of paint is supposed to work. I've just used bare wood with a little oil sprayed on it, too.
5. Step Five: With some nice cloth gloves to protect your hands, use the heat gun to begin shaping the canopy to the form. The pop bottles shrink a considerable amount, like Monokote on steroids.
6. Step Six: Trim and glue to your plane.

Of course, vacuum-forming is a lot less error-prone. With the above process, you're likely to mess up a few times and stretch or wrinkle the plastic. No biggie, your local Wal-mart probably has an unlimited supply of plastic for you :)

You can also re-use packaging from other vacuum-formed items if the pop bottle plastic is too thick. The stuff left over from purchasing tools works well for smaller canopies, and many brands shrink akin to a shrinky-dink when heated.

Wade Joos has done a lot of parts fabrication, but I think most of his work is fiberglass & epoxy in a mold with release agent. For a canopy, not exactly what you want.

I need to replace the canopy and cowl on my old Great Planes Patriot 40, and will be doing this same thing at some point this summer. Great flyer, looks horrible.