My #1 supplier for computer parts. I've probably spent over $4,000 on parts over the past decade. In addition, for teenagers looking to get started on positive credit history AND are responsible enough, you can also get set up with a credit account on your newegg purchases. Just be sure to pay it all off before the 0% interest rate expires (<$500 you got about 6 months to pay it all off, >$1000 you have a full year. Be sure to review and read the credit terms before going through as it is subject to changes).
Still, a good PC parts supplier to at least price parts and read reviews on anything you're interested on buying.