I've replaced a few and I use bearing units from Kaman Bearing, which is a local industrial bearing outfit. They are quality pieces with SKF bearings. Not bad prices compared to factory stuff. Bea ready to buy all the seals which are not cheap...around a 100 bucks a side and you'll need a special tool to install the main seal. I borrowed the tool from a friend who works at the dealership. When you do this job, seriously consider replacing the axle u-joints with greaseable u-joints. This will save you from buying seals twice as the seals are a one time deal. Some places have a seal "kit" like powerstrokeshop.com. I used dealer seals as I needed them right away.
The Hauler - '05 F-350 4x4 CC Dually 6.0 Powerstroke
The Project - '90 Ford Bronco 4x4
The wife's - '11 Ford Escape
For every minute you are away from work...you can add two minutes to your life!