Retailers like Party City offer a wide selection of foil and latex balloons with helium inflation included if purchased online for in-store pickup.
If you have already purchased balloons elsewhere (e.g., specialized designs from Amazon), some stores provide inflation services for a fee: Balloons SEO Overview Text - Party City buy inflated balloons
Many local grocery stores (such as Kroger, Safeway, and Vons) have floral departments that sell and inflate balloons. Retailers like Party City offer a wide selection