Bearings on Tow Dolly. Where to get? Any tips?

Can someone who has done this tell me if you can pick up bearings for a tow dolly at the local auto parts store? Or is this something that I have to go to the trailer dealer to get?

Also, is there a number somewhere on the actual bearing that would tell me the size? Or do I need to take the old ones with me when getting new ones?

I am somewhat handy and planning to do this myself. Have never done this so any tips or suggestions would be appreciated.

Thanks in advance for your help. Now to get my hands dirty.

Moved to Tech Issues forum from DIY.