I can't directly address the question, but it's a fairly common part, not a specialty item. Take one off and take it to the hardware store. You might have to go to more than one place, but some parts retailer is bound to have them. If you can't find them with the exact same bolt pattern, it's sometimes possible to buy ones with the same size wheel and replace just the wheel portion of your old casters. If you're replacing the whole thing, it's worth considering using through-bolt hardware (with fender washers on the inside) rather than lag screws.
