Most gear sites won't tell you this because it costs them money, so here it is plainly: if you're doing the Narrows once, rent.
The Springdale outfitters rent complete packages — canyoneering boots, neoprene socks, a walking stick, and bibs or a drysuit in colder months. It costs a fraction of buying, the boots fit properly because someone sizes you in person, and you don't have to fly home with soaked gear in your luggage.
Buying makes sense if you're hiking the Narrows more than once, doing other canyon routes, or already do wet hiking elsewhere so the gear won't sit in a closet. It also makes sense in shoulder season, when you want a wetsuit that actually fits you rather than whatever's left on the rack.
The small stuff. Neoprene socks, a dry bag for your phone and keys, quick-dry layers, and a headlamp are cheap, pack flat, and get used on other trips. Rent the boots and bibs; own the rest.
Springdale sits right at the south entrance. It's where you go for Narrows rentals, last-minute forgotten items, and anything you need the morning of your hike. Selection is narrower and prices reflect the location, but you're minutes from the trailhead.
St. George is about 40 minutes west and has the area's biggest selection. If you're arriving the day before, need real footwear options, or want to compare more than two of anything, it's worth the drive. It's also where you go if Springdale is out of your size.
The "Gear run" button above routes you hotel → shop → Zion Canyon Visitor Center, so you can pick up gear on your way in rather than doubling back. That matters more than it sounds, because of one constraint most visitors don't plan for: for most of the year you can't drive into Zion Canyon at all, and once you're on the shuttle you can't return to the car. Whatever you didn't buy, you do without. See the shuttle guide for the details.
Rental shops open early — often before the first shuttle — specifically because Narrows hikers need gear before 7 a.m. Call ahead in peak season; the popular boot sizes go fast.
An Angels Landing permit. Those come only from the Recreation.gov lottery, no exceptions, and no shop in Springdale can sell you one regardless of what a sign says.