I think the "standard" choice people make are the Kuryakyn Iso-grips. Available in black or chrome, I believe.
ISO®-Grips | Grips | Handlebar Controls, Grips & Mirrors | Kuryakyn
You want the model that has an integrated throttle tube, not just the grip to put on the Triumph tube. Much easier to replace the whole thing than to try to wedge the grip onto the existing tube.
Avon also makes a nice assortment. They also offer theirs with heat built-in, I believe they use the Heat Demon element which is nice.
Motorsports V-Twin Grips - Avongrips.com
Anything with a 1 inch diameter will work, though, basically.