Go to a trailer store, tractor store, or Academy or Bass pro shop. Bring one of yours, and match them up.
Shouldn't be much of a difference between brands and if I am not mistaken, there are only 2 or 3 manufacturers of bearings.
Oh, walmart may even have them, depending on the size.
Get new bearing buddies while you are at it.
If they are really bad, you can get entire hubs with bearings already installed at some of these places (academy and bass pro)