As a woodworker, I buy my shellac from places like Woodcraft in the form of flakes. The most important thing about shellac is that it must be fresh. Shellac that is more than 6 months old may not harden properly. Zinser pretty much has the corner on the market in the shellac world. You can buy it at most hardware and paint stores pre mixed in cans. It has some sort of stabilizer in it that extends the shelf life. The catch is that they don't put dates on the cans and you have no way of knowing how old the stuff is. If you want to use the premixed stuff, I suggest you buy it from a place with high turnover. Check it on a piece of scrap. If it dries quickly and hardens without staying tacky, it's probably ok to use. You can chose from either orange shellac or various lighter shades which is pretty much the same stuff with the some of the impurities processed out of it.
The other issue with shellac is that it should be dewaxed. The wax is not really a big problem if you are not overcoating the shellac with another finish. On a canoe bottom, I doubt that the wax would give you much trouble.
If you want to mix your own you can buy dewaxed shellac flakes. Jeff Jewitt sells the flakes on his website which is:
http://www.homesteadfinishing.com/. You just mix the flakes with shellac thinner (alcohol). I estimate a four pound per gallon cut by putting the flakes in a jar and just covering them with alcohol. Let it stand overnight and agitate it occasionally. Then dilute again with an equal volume of thinner and you'll have about a two pound cut which is about right for finishing. (Put a date on the container so if you want to use it again you'll know how old it is.) Use a good brush and work fast as the shellac dries quickly. My quess is that plain old orange shellac would work just fine for you.
If you buy flakes that are not marked dewaxed, you can still mix your own. Just let the mixture sit a few days. The wax will settle to the bottom as a milky looking precipitate. Just siphon or decant off the liquid above the cloudy stuff and you'll have a dewaxed mixture.
Hope this helps. By the way, in answer to your question about whether there is special shellac for this application, I believe shellac is shellac. It's a secretion of a bug. There are various levels of purity, but any of the ones I mentioned above should work just fine for you.
