If you're on reddit theres a section called /r/hardwareswap that always has a *ton* of cheap stuff, some even local for pickup. Microcenter, offerup, and ebay are also pretty decent.
A great site to check out is pcpartpicker.com if you haven't already found it, it'll measure wattage, possible fitment issues etc
A great site to check out is pcpartpicker.com if you haven't already found it, it'll measure wattage, possible fitment issues etc