There are a few things I can think of.
Bricklink is a reputable online site. You can buy any pieces ever made by LEGO across the globe. Their site may seem hard to navigate at first, but it doesn't take much to get used to. Also, if you have any questions you can ask here.
2. Pick a Brick
If you live near a LEGO store you can find yourself a variety of parts. The prices are decent. Don't be afraid to fill the cup a little more than full. They have tape just for that purpose. The selection changes as they run out of parts.
3. Local LEGO Shops
There may be a small locally owned LEGO establishment near you. I am not talking about a supermarket like Target or Walmart. I refer to small "holes in the wall" that may have a pick a brick much like LEGO Stores.
4. Bricks and Pieces
This is a suggestion that I myself have not used personally. Apparently you can order any parts they allow in production in your desired quantity. I am not sure of the price, but the reliability should be there since it is from LEGO.