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.
i can tell you right now...ive been riding quads for 6 years before i recently switched back to dirtbikes and thumb throttles SUCK!!!!! they get extremely tiring for your thumb till the point where you have to use your wrist. it may bey cool and different but i would definately would not reccommend it!