Basic outdoor stores like REI and Eastern Mountain Sports might have a small amount of canyoneering gear. Just because the gear isn't labeled for the activity doesn't mean it won't work. Many of the same pieces of equipment you would use for rock climbing, mountaineering and spelunking are also used for canyoning.
Depending on the difficulty of the canyon, gear could include harnesses, helmets, wetsuits, dry suits, rope, carabiners, rappel devices, gloves, elbow and knee pads, webbing, biners, prusiks, descenders, survival kits and canyon dry kegs or water-resistant packs that keep your belongings dry when you're in the water. If you're new to the sport, contact your guide in advance to make sure you'll have all the items you'll need for a specific trip. All canyons are different, so the equipment will vary