Small o-rings in color

If you want them *RIGHT NOW*, check out a place that does piercings or sells different types of ear gauges and whatnot. They usually have a huge variety of colors and sizes of o-rings. Otherwise, you can check any place that sells those types of things online to find a decent price.

Lighthound.com sells pretty decent GITD ones (better than a lot of the ones used for body art), but they have super high shipping if that's all you want to buy from them.