The Mophie Juice Pack does the job. There's also a separate belt clip that you can buy for an additional $20 that works with the Juice Pack. The belt clip's compatible with all 3 iPhone 5 Mophie Juice Packs (Helium, Air and Plus). Personally I'd just say get the Juice Pack Pro if you're a heavy user. If not, get the Air.
Also, since you're in the US (not sure if it goes for any other countries), depending on the carrier, you might be able to get a 10-20% discount on one of the Mophie Juice Packs if you buy it in your local retail store (not available online, but you might get a discount at your local carrier store if eligible/applicable). That'll save you a few dollars that you can either spend on the belt clip, or spend on upgrading from the Helium to the Air, or the Air to the Plus.