What brand are they? You might contact the manufacturer and see if replacement grips are available.

Would golf club grips fit? (Take the poles to a golf store, like Golf Galaxy, that does grip replacements, and they can tell you.)

However, in the end, your best bet might be to shop the outlets for new poles. Sierra Trading Post, REI Outlet, Campmor, Moosejaw, and Backcountry Gear often have really good sales on poles. Hit the right sale, and it might be cheaper to get new poles than to repair the poles you have. (Sad commentary on our disposable culture, isn't it?)