Tournament programs, if available. I had/have a pad of black and white table tops with the autographs in the center, about 5"x7" IIRC. Magazine covers are good. Pictures cut from magazines, with enough room to frame at the sides. The 1993 World Wide Collectibles, Inc. set, if players from the set will be on hand.
These have very, very, little value otherwise. Black and white or color 8x10 photographs, you can usually buy these from a vendor at major events.
Books authored by players, there are quite a few authors/players who attend major events, Ewa Mataya Laurance, for example.
Cue balls and/or numbered balls.
Cues, though not necessarily expensive ones

3x5 index cards, lined or unlined.
Cases, though I'm not fond of this idea, myself.
VHS/CD covers.
Tournament posters.
You are only limited my your own imagination. I collected autographs, though not just of pool players, for several years and it's amazing what collectors will come up with to get autographed. We are going to the Open, so I'm hoping to get a bunch of new ones, myself.
Good luck!
Edit: I'm pretty sure Jay Helfert woud be more than happy to sign any unsigned copies of Pool Wars! WOW! Glad I caught that typo!