Designing a circuit board is only part of finishing a project. Finding components is probably one of the most tedious and mundane tasks. Adafruit put together an article and identified some sources for finding parts. This will come in handy in those situations where your primary vendor is out of a specific component you need. I usually purchase my parts from either Sparkfun, Digikey, Jameco, and/or Mouser.
Read about it here: Adafruit: Finding Parts