I use Walkerswood Traditional Jerk Seasoning. It is one of the closest I have found to authentic. You can find it in West Indian food stores as well as the ethnic food sections in some grocery stores--I believe I have seen it in No Frills.
Walkerswood is pretty good. Also Winston's Jerk if you can find it. As much as I love to promote Taste the 4th Sense on the Danforth - their prices are way over the top for Walkerswood. Try Kensington or even Wal-Mart believe it or not.