I get some of my stuff here
Electronics Supply Co., Inc.- Kansas City, Mo. but they are local to me so I walk in and look all around.
Then Microcenter has a hobby electronics department that has been growing steadily for the past couple of years. Again, they are local to me so I cam go in and look around.
Boards/Projects : Hobby Electronics : Micro Center,
Electronic Components : Hobby Electronics : Micro Center,
Electronic Parts : Hobby Electronics : Micro Center
And then online I have used
https://www.sparkfun.com/,
http://www.adafruit.com/ and a couple of others.
I also use
https://www.pjrc.com/. I have several versions of the teensy processor I work with.
.