Marathon Easy Fit Pneumatic (air-filled) 4-6 Tire and Wheel Assembly, Wheelbarrow/cart Use, Ribbed Tread in the Wheels & Tires department at Lowes.com
Jackson Spoked Wheelbarrow Tire 15-1/2 in. Dia. x 6 in. W Butyl Rubber | eBay
Amazon.com: AR-PRO (2-Pack) 4.00-6" Tire and Wheel, 13" Pneumatic Wheelbarrow Tire and Rim, 5/8" Bearings and 1.75" Centered Hub, for Garden Carts and More (Extra 3/4", 1/2" Bearings and 1.5", 1/2" Nylon