A combination of Micro Center, FS.com, and eBay for computer parts. Anything worth researching I'll try and buy direct from the manufacturer.
Dumb cheap stuff still goes to Amazon because if I need a $2 female USB-B to male USB-C adaptor or something like that I'm not willing to go through more than about 5 mins of searching and I know there's some random blob of letters company on the Amazon marketplace that will give me something that functions. I definitely wait until I have something critical or reach $30+ before actually placing the order though.