My first build was over 15 years ago and I got everything at Newegg. I think I may still have that system, running Windows 2000. I used it a lot the last 2 months because the more recent build went down with a blown PSU (replaced over the weekend).
I'm rebuilding those two systems shortly, will shop components. I sometimes look at ebay, but usually find better prices at Amazon and who would you rather trust?
I've bought from Tigerdirect, B&H too. It all depends on who has the better deal. I don't have Prime, but rarely pay Amazon shipping, just make sure my order's over $35. If it's not much below $35, I have gone to
Filler Item to find something hopefully useful to get me there.