Bricklink.Com, then click on the Search tab and select Find Stores. Now, click on the --- All Colors --- dropdown menu and select your color of choice. If you're planning a smaller order and are looking for a color that isn't overly rare, you may want to limit seller location to your country of residence. Except for you, lonely Bricklink user in Fiji.
If you can't figure out the specific color you're looking for, or just want to find something really rare, take a look at Bricklink's color list. Personally I recommend Dark Azure, Magenta and Bright Light Orange.
This was just a quick tip which someone shared with me a couple years ago and I've found useful. Let me know if there are any other advanced Bricklinking type tutorials you guys would like to see!