I've purchased them at the big box garden, home supply, hardware stores. Home Depot, Ace, Lowe's, nursery, etc. Just be aware of the possible rust issue on some on the metal wheels. And, if you put WD-40 or rustoleum on the wheels, don't do it on the patio. It will stain the concrete. Spray it off the patio and don't put it back onto the patio until it's totally dry. No dripping or seeping.
