Go kart shop...cycle shop, and if yer really in a hurry ?
Some bicycle brake cables will also fit, if the "bead" at the end is the proper size.
Pull the cable and haul yer arse to the Walmart/Meiers/OtherEvilSuperstore and go to the bicyle aisle and see if you get lucky.
As for how to put it on, use the SEARCH command and look for "Tomos Boatload" where you'll find instructions for how to get the carb out.
Best way to attach the throttle cable is to slide it out of the sheathing, and run the non-beaded end up through the "noodle" (you'll know it when you see it) and then put the sheathing back on.
Here's an pretty bad ASCII of how that cable hooks up in there, I think...it's kinda tricky till you get used to it.
At the throttle end ? it's pretty self-explainatory, lookin at it and all, but if you have any problems, just sing out and I shall be along shortly.