It's like turn signals, tires, tires, chains, mirrors, master cylinders, grips, bulbs, fuses, bolts, connectors, bearings, and 1000 other parts on your bike, as in not very bike specific if you know the measurements for what you are looking for.
Outlaw is right, because what you want is a "7/8" (22mm) dual cable throttle tube". Probably $5-$15. 99% will slide on perfectly without any mods and last another 20 years.
What you don't want: is a R/6 conversion (bigger), '1/4 turn throttle', fancy race stuff, outthinking the non-issue of a 20 year old plastic thing failing, or searching for a "Buell XB" anything that will end up costing you $180.