You could probably get one at a Snapper dealer, small engine repair place, and have the throttle cable cut to size.
The best thing about a thumb throttle is you can make a "cruise control", mine is a piece of clothes line, wrapped in red electric tape, twisted around the handle. When out in the open, I just slip it over the throttle to hold it down, frees up the right hand.
Others have made a cruise control out of a large rubber band.