The "J" tube (for lack of a better name) you are looking for typically comes as part of a motorcycle throttle cable assembly.
I was unable to find just a "J" tube anywhere when I built my trike last year.
Since I'm running a HD 1" throttle, I went on Craigslist & found a guy that upgraded his stock black cables to stainless style. I bought one of his cables & used the 33", +or - cable outer jacket that included the elusive "J" tube, & joined the HD cable to one I purchased from go kart supply. (Stinger has their link posted above)
I used electrical heat shrink tubing to "marry" up the outer HD cable to the outer cable housing from go kart supply & used about 8~9 feet of the 12' go kart supply inner cable to reach the engine.
Sounds like a lot of work but it really isn't, & it works like a charm.
If you're running a "metric" throttle, the same "J" tube part should be readily available on Craigslist too.
Good luck & let us know when you get it solved...