A lot of the extentsions you see on the internet are for trolling motors and not suitable for outboards. I use the Davis Handi-Mate extension handle on my 18hp Evinrude and it works fine. You can see it in the pic below.
Google "Handi-Mate" and you will find a bunch of on-line retailers you can buy it from. Whatever you get make sure that it is rated for an outboard motor and not just for use at trolling speeds.