You can't really expect to buy parts and re-use for projects forever. While learning, a solderless breadboard and re-used parts are great, but for real projects, the parts are pretty much consumed by the project.
You *can* buy various part kits on Amazon, and a box of resistors and a box of LEDs might be a good investment, but most projects need more specific parts, too. I generally save up my needs on a list, and then place an order with Digi-Key once every 2 months or so. That way, the postage ($5-$7 for first class mail) ends up amortizing over a whole number of parts. Digi-key is great for "raw" components -- MOSFET transistors, Atmega328p microcontrollers, specific value capacitors both ceramic and electrolytic, etc. And when you buy one of something, you typically want to buy three, because you'll destroy the first one when building, and the second one will burn out while debugging :-)
Note that a "raw" microcontroller also needs some way to program it, as they don't come with nice serial boot loaders like an Arduino. A decent AVR programmer can be had for under $20 from places like SparkFun or Adafruit.