When I rebuilt mine, I found the best prices at etrailer.com and the quality is great. I ended up having to rebuild two of my four wheels with new bearings, races and seals and etrailer had the kits where you get all that at under $20 a wheel. This gets you a new tang washer, two bearings, two races and an inner seal.
See this link: http://www.etrailer.com/dept-pg-Trailer_Bearings_and_Races-sf-Bearing_Kit-pc-5200_lbs_Axle.aspx
If you can take the old bearings/races out on one wheel, you can sometimes still see the part numbers on them to order the replacements (I think they use a standardized designation). If not, you can still use them to take measurements to be sure you are ordering the right parts. The style of the nut retaining system can also vary (cotter pin, tang washer or retaining washer) so disassembly will also tell you which style of retainer you need.
Once you drive out the old races, keep them and make a radial cut through the old race with a cutoff wheel on an angle grinder. Then you can use this to drive in the new races and the kerf of the cut lets the old race compress to a slightly smaller diameter so it can be easily removed after the new race is driven home.
etrailer also seemed to have good prices on the electric brake kits for trailers as well. The only thing I found I could get for a more reasonable price locally is brake shoes.