I've seen a generic mast boot kit available at most boating stores. The kit has a rubber boot and glue (should the boot need to be slit if the mast already is stepped.) You provide the hose clamps and trim it to size.
I've had mixed luck with one of these. The second year I used it, I got a lot of water in the boat whenever it rained. This may have been due to a hose clamp not tight enough. I'm probably going to try putting some sealant on it, and if that fails I'm probably going to break down and buy SparTite.
