A-788 is made by Pettit Marine. Try a marine supply store, especially one that services commercial marine. It's used a lot to patch steel boat hulls.
I don't know what you have in Canada, but West Marine, here in the South, has it. If mail order works for you, Jamestown Distributors in Rhode Island has pretty good pricing, Quarts to Gallons.
https://www.jamestowndistributors.co...ct.do?pid=6766