Since you've already been to Glacier and Yellowstone NPs, I am only aware of two other NPs in the lower 48 where you might encounter the Grizzly: Grand Teton NP (moderate risk) and North Cascades NP (very low risk).
Grizzlies can supposedly be encountered in any Alaskan state or national park. Rifles and shotguns are allowed in Alaskan state parks, and you are allowed to shoot in self defense. Any bear shot and killed in self defense must be salvaged and turned over to AK Dept of Fish and Game. For details see http://dnr.alaska.gov/parks/safety/bears.htm
For perspective, I used to run a cattle ranch at the base of the Grand Tetons, well within the range of the Yellowstone Grizzlies. I NEVER went into the high range alone without a loaded firearm of sufficient caliber. Fortunately, I never needed it, which was fine by me.
Seasoned bear hunters have told me that if you shoot a Black Bear, the odds are 50/50 he'll charge. With a Grizzly, it's a 100% certainty. Better to use a good SLR camera with a long lens. If all else fails, hit him with it!