When I change out parts I like taking them off first and bring them with me to the parts store so they can not only see it themselfs they can also get the number off it. If it is a more common part such as brake pads/shoes or brake rotors/drums I just get the numbers off them and bring them in. A place like napa isn't much help here unless you know if the number is a Spicer, Dana number or another company number.
This is what part of the country you are from helps us help you better in finding parts that you need and where to go local to your location. I my area southern minnesota http://www.catcoparts.com
is a good one to go to and the Burnsville guys there showed me a cheap part to fix my problem I had when my rear wheel clamps on the right side were not holding the dayton rim tight. Something they called a cheater ring. Which was a 20 inch ring to act as a spacer to help the clamps hold tight. The thing worked great for less than $10.