Depending on where you are & what retailers are near to you, microcenter is probably one of the best, if not the best one, however, they only have stores in certain areas, but if you have one close by, then you're golden, as they have just about anything & everything needed to build a pc, and are well known for the mobo/cpu combo deals that can save you some serious coin...
BUT be advised that they do NOT ship most of their stuff, with a few exceptions...
If buying stuff online, then Amazon, B&H Photo, BustBuy, and for certain things, even Wallyworld has some great prices and fast shipping too...
I know I'm gonna upset some folks here, but I strongly recommend that you do
NOT buy anything from NewEgg....they used to be great but got bought out by some crooked foreigners, and went to shit, with near zero customer service, crappy/slow/unreliable shipping, and really shady/questionable suppliers....
And as for in-person visits to BustBuy STORES....not no, not heil no, but F&CK NO ! You'd do better by cutting your genitals off & eating them for lunch....
You can however, use their search tools to find specs & prices for things, then go elsewhere to buy them
Also if you want to build a hypothetical/mix & match rig (or 6) & see what it's gonna cost and if there are any compatibility issues, then head over to pcpartpicker.com and have a blast !