Tractor Supply has a great selection of trailor parts.
I always take in the old part when purchasing a new one regardless of which store I go to because When you take your old part in you are sure to get the correct replacement. When you don't, Murphy will send you home with something regardless if it is right or not.
Tractor Supply Company - Stores by State
ATWATER, BAKERSFIELD, CHICO, CLOVIS, DIXON, GALT, GILROY, LOS BANOS, OAKDALE, OROVILLE, PETALUMA, RED BLUFF, REDDING, RIPON, SONORA, TULARE, TURLOCK, UKIAH, WINDSOR, WOODLAND, YUBA CITY
Walmart also has bearings on the rack.
Take your grease seal with you. It should have a number on it also.