well... until i even had a twist grip... i had a brake lever for a throttle.
however, i just recently upgraded to my first and only twist grip. it is a twist shift from a mountain bike. it works well, is smooth and seems durable enough... and probably already has a cable.
i do however recommend taking out the spring loaded pawls that lock the grip into each gear... for mine, that was as simple as taking out one little piece of spring steel.
thats also prolly cheaper than buying them too.
but if u really wanna buy one, i heard somewhere on the net of billet twist grips... lemme try and find the link...
here ya go...
http://www.overstockdistributing.com/product.php?productid=617&cat=0&page=1
also, try typing "twist grip" into ebay, that yielded quite a few legitimate results for me...
good luck!
BAMBY