You can find bearing seals at most auto parts stores, bearing shops, and RV supply places. They are common and prices are all over the map.
It is always a good idea to take an existing bearing or seal with you when you go for new parts. Often they have part numbers stamped on them but if you hit someone who knows the business, he can spout part numbers on sight and grab what you need. Seals, especially, are often measured just to be sure.
6 bolt 5.5 inch circle Chevy? That's how Low Cost Trailer Supplies used to title them - of it's the same hub as used on the Airstreams. That website seems to be down, now, though. They had single sided double lip seals for $2 ea. I've seen similar at NAPA for $15.
An RV parts place may have your bearing sets in a shrink wrap package for $20 or so. These can be handy for having on hand as they include both inner and outer bearings, races, and grease seal.
You can get seals that are single or double sided, single or double lipped. For trailers, it doesn't make much difference except as a factor in the cost.
Replacing the seals is cheap insurance. getting them out can distort them as they are press fit so replacement helps assure a good fit.
Inspecting the bearings means you need to clean them well and that is good as it gets all the old grease out. See Bearings
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