The caps with zerk fittings are a bad idea, IMHO. They make it way too easy to push too much grease into the hub, and when it gets hot it slings out of the back seal and all over your trailer brakes. They are good if you monitor it regularly, but you still end up having to tear down and repack by hand once a year. My trailer came with them installed, and I used them at first, but discovered the hub filled entirely with grease and it destroyed a set of brakes, so I just do the repack manually every spring.
The best online source I've found is
www.e-trailer.com They have most every type of trailer part you could want, but nothing beats doing a little cross-referencing of you own at NAPA to get parts locally. I was able to get bearings, seals, and races at NAPA for way cheaper than any of the trailer supply places.
Depending on what type of trailer you're dragging, an oil bath might not be a bad idea, but I've found traditional grease and tapered bearings to be very easy to deal with. In my experience, the folks who have trouble with them either A) overload the trailer and or lend it out to people who don't treat it right or B) are a little slack on maintenance and inspections...
Ryan