Sig sells all kids of aftermarket canopies...

You should be able to contact the distributor of your airplane's kit and get a replacement. $4 to $20 for .40 to .60 size planes... depends on the canopy.

Many airplanes, you can cut up a 2 liter soda bottle and make a canopy without any heat forming needed. (you can get a nice green tinted canopy from the lemon-lime soda or Mountain Dew bottles...)