I've ordered brakes and bearings from etrailer.com, and I recommend them as a source.
I got self-adjusting brake assemblies, rather than the ones that needed manual adjusting that came on the trailer. They've worked very well. As far as size goes, you can measure the diameter once you take the hub apart on one of your wheels. I think that's the dimension you need. I believe mine were 10", but yours may be different. Get the whole assembly, not just the brake pads.
I think I measured my bearings too, (just the outer one) with the hub apart. I would not get the "bearing buddy" kind that are supposed to make lubrication easier. Everything I've read about them says they don't work well and are not reliable.
If you take the bearings out of your wheels and inspect them, if they are not scored, they can be re-used. They just need lubrication with bearing grease. You WILL need new seals though.
You CAN get the bearings from an automotive store, but etrailer has good prices and they sell quality parts.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew