Assuming you mean digital circuitry (99% of what's built today), I second the Adafruit recommendation. Sparkfun has good stuff but their focus is more on experimenters whereas Adafruit focuses on education. Their tutorials are very well done, and their hardware is much better "polished" than Sparkfun.
I point out to people that it's clear that Adafruit is noticeably more expensive than the cheap stuff on amazon, but the fact that you can buy something from them, follow the tutorial on it and basically expect everything to work right off the bat makes the cost worth it. Even as a professional embedded developer, I go straight to them when I need to build a prototype and don't want to waste time faffing around.
I point out to people that it's clear that Adafruit is noticeably more expensive than the cheap stuff on amazon, but the fact that you can buy something from them, follow the tutorial on it and basically expect everything to work right off the bat makes the cost worth it. Even as a professional embedded developer, I go straight to them when I need to build a prototype and don't want to waste time faffing around.