You could buy a pint from any automotive store. It is called limco 1 which is a straight enamel. Appliance paint is the same thing I think it is more expensive tho
I understand being cautious about poisoning fish with toxic chemicals in paint, but I really don't see the need for spending extra money on special paint....especially for a canopy. My stands and canopies have always been painted with either a gloss enamel or acrylic. I stay away from any latex paints because they just don't produce a professional finish. Acrylic enamel and straight enamel is going to offer the best protection and if preserving the wood is a concern, a clear wood sealer can be applied first then any color after. If you use an acrylic enamel, don't put a straight enamel over the top or it will wrinkle.
Not trying to belittle your concerns and I certainly appreciate your effort to protect the fish, but I just don't see the need. I see people painting decor and plastic parts to go inside the tank with Krylon Fusion...a paint made for plastic...but if the fish eat it or scrape it off like a pleco can do, they are still injesting chemicals not natural to their diet.
There is a greater danger of your fish getting something deadly in a water change than from the paint you use on your canopy. The silicon used to seal the tank is more of a concern in my opinion.
I have done this for many many years and have never seen an issue with paint other than the use of industrial grade apoxy resin type paint...but even that paint, a paint you thin with KETONE, once dry, gives off no fumes or chemicals.
Again, not trying to negate your concerns....just trying to put you at easy a little.