There's a topic that lists the stores in their perspective country. https://forums.d2jsp.org/topic.php?t=67028064&f=189
I mainly use NewEgg, Amazon, MicroCenter, and BestBuy in that order. Keep in mind, I'd like to think most of us buy computer parts and build it ourselves. We normally do not buy pre-built.
I think pre-built computers are better for you. Other companies that does pre-built would be like Dell/Alienware, HP, Lenovo, etc. If you want more custom options, then try looking at CyberPowerPC, iBuyPower, Maingear, etc.