What you're looking for is something like this:
http://www.ledlight.com/detail.aspx?ID=39
However, I advise agaist using those aftermarket LED bulb-replacements (especially if you plan on replacing your brake lights). They do NOT give off the amount of light as your original bulbs. The optics within your tail light's housing is optimized for regular bulbs that give off light in 360 degrees. Even those omni-directional LEDs don't cut the mustard. Where that becomes an issue is in an emergency and you NEED to be seen (via your hazard lights).
When LEDs come from the automotive manufacturer, they're grand. Otherwise you I suggest you play it safe. Sorry to be a wet blanket.