Chief Delphi - mecanum wheels purchase

I would just call them up and get the scoop. Often they know several days in advance when they will be getting new parts in from the vendors and can estimate when they might be ready for sale. They periodically get stock in throughout the build season on stuff like wheels to meet high demand. The staff is helpful over there and will do their best to hook you up.

To answer your question more directly, sometimes hobby/competition robot supply sites like Robot MarketPlace will have their own versions, but they are usually meant for smaller, lighter-duty applications. Hold out for the AM ones if you can, the customer support that is on-site at many of the events alone is worth it!