I was messaged by a fellow AFOL recently about were to buy indervidual bricks. For any one making a former set or looking to build MOCs (My Own Creation).
Here are just a few of the places to look when trying to buy specific parts.
BRICKLINK
Brick link is a community site of like minded LEGO fans. Ultimately these are inderviduals that run a business selling LEGO parts sometimes you can fund a bargin but its a little hit and miss in my opinion.
TOYPRO
Toy Pro is an EU bassed retailer that has a vast range of LEGO sets and Minifigures. In addition to this they have a brick picking and buying service.
Mobile site
Desktop site
Brick Pick
LEGO PICK A BRICK
And ofcouse there is the LEGO Group themselves. This is a good starting point but it is not always fully stocked. This is the best place to start and then shop about.
Mobile Site
Desktop Site