Depends on what it is, who made it, where it was made, how much it costs, how good their customer service is, how reasonable their shipping fees are, and how long it will take them to get it to me.
Me being a cheap-o buy used whenever possible. A lot of times, someone will buy a part and never use it and unload it for cheap. Mainly classifieds on forums and craigslist.
Depends on what it is, who made it, where it was made, how much it costs, how good their customer service is, how reasonable their shipping fees are, and how long it will take them to get it to me.
I figure out the part I want, then find who has it the cheapest. And I'm impatient so sometimes shipping speed has more weight than cost, just depends.
Depending on the part and not in any particular order
U Pull it
Classifieds
CL
Ebay
Summit
LMRS
American Muscle
Rock Auto
Lately I've been finding CL is too much hassle. Too many crack heads and scammers who either want near new prices for used parts or are selling parts that are broken or hacked up. EBAY is ridiculous most of the time- people think because it's on EBAY you can ask XX times what something is worth.