Remove both an inner and outer bearing on one good wheel and clean them off. You don't state where you are currently located, but, find a professional industrial bearing supply company close to you. (Google is your friend) Take the old bearings there and tell them you want new bearings of equal or better quality than the ones you have . . . . . they can help you with getting the right stuff. They will explain your options. (manufacturer, country of origin, ABEC grade, etc.) You and your budget now determine the quality you get, not what happens to be on the shelf. IME, if you find a good vendor, generally, these guys aren't the minimum wage pimple faced millennials that you'll find at national chain auto parts stores. They KNOW bearings. It's what they do. IME, many times, its actually cheaper (and better quality) than going to national chain auto parts stores.
If you are stuck in the boonies, McMaster-Carr can help you by phone, online, and by mail order.
Most roller bearings are very common interchangeable parts, . . . if you know where to look and how to ask for help.
Chum lee
