Have you looked in your phone book for boating supply shops? Chandlers?
Don't remember seeing any when I was in Kona, but there's enough boats there that there has to be at least one, probably a few. Barring that, you can probably get away with using automotive 90wt hypoid gear oil (differential fluid.) Marine stuff would be better, as it contains a bit more anti-corrosion stuff (at least that's what's rumored.)
A Google search for your Merc model and year (and 2/4 stroke) will get you a few places that will sell you a download of the owners manual and/or service manual. You can also get them through the Mercury Marine website (but have deep pockets.)