Any aftermarket grips for a 1-inch bar will work. They are very easy to install, but if you get universal fit ones instead of Intruder-specific grips, you may need to destroy your stock grips to re-use the original throttle sleeve. I just put on a set of ISO gell grips on one of my Intruders, and it took about 20 minutes, including adjusting the throttle cable afterwards.
www.accessoryinternational.com has an asortment of Arlen Ness grips.