After searching area suppliers and the Internet, I found a pair of suitable 8-inch pneumatic wheels with ball bearing hubs for the wheel barrow handles. I painted them with Krylon glossy black enamel paint, then made two bumpers that act as spacers from leftover Brazilian cherry I used to build my dresser. First I drilled a 7/8th-inch hole deep enough to mostly accommodate the lock nuts, then I drilled a 7/16th-inch hole the rest of the way through.
After three coats of polyurethane, I bought some stainless steel 1/2-inch all-thread, 3/8 and 1/2-inch washers, and lock nuts. First I ran the hex nuts that came with the eye bolts all the way to the head of the bolts. Then at each hole in the handles I slipped a flat washer onto the bolt, inserted it through the handle, then the wooden bumper, another flat washer then the lock nut. A socket wrench and deep well socket was then used to drive the lock nut all the way down the eye-bolts until it's just short of tightly holding the bumpers to the handles. The eye-bolts and bumpers are now captive on the handles and will not fall off and get lost in the grass.
Attaching the wheels required first the sawing of two 5 and 1/2-inch sections of the half-inch stainless steel all-thread, then filing the ends to accept lock nuts. Two lock nuts hold the all-thread onto the handle. Next a stainless steel flat washer, then the wheel, then another flat washer followed by a 1/2-inch Nylon insert lock-nut secures the wheels in place on the handles. A pair of 3/4-inch open end wrenches dogged the nuts tightly in place but allows the wheels to turn freely.