Honestly, Amazon is perfectly fine. Or if you’re lucky enough to have Micro Center near you, that. PCPartPicker checks multiple stores, so that’s not a bad place to start, at least for current and historical pricing information.

I’ve never used it, but I know people who’ve used Facebook Marketplace for used computers/components. I’ve used eBay before plenty (even sold parts there).

Edit: You can also go direct to some manufacturers. EVGA often sells brand new and sometimes “B-Spec” components, often at a good price. I purchased a brand new PSU from them directly and got a great deal on it.