I'm sure many of the vendors sell replacements. Pick one of the site sponsors for a direct replacement.
Or you could go to a local dirtbike shop and buy a replacement. If you are feeling flush you can even get one with an AL tube vice the plastic tube. Nice upgrade on a dirt bike probably overkill on a MB.
As for a cable take your old one with you and you should be able to get one at a well stocked shop. Most of the Honda 50s and such use a similar tip and the MC ones usually have an inline adjuster on the cable to give you a nice tight throttle with no slack.