Since this kit was recently released, go to www.revell-monogram.com and request a replacement canopy. There is a $3.00 "handling" charge on each request (up to 10 parts per request, 10 requests per year).
Another route is to write them at the address on the instructions for the replacement. As I understand it, since they do not state there is a $3 charge for parts replacement in writting, there isn't one if you write.
HTH
Jeff