Same bulbs as the side markers.
These are the ones I purchased.
I'm thinking of getting these for the 3rd brake light, since they use the same bulb as well. But I think it might be too bright, cause the side marks are REALLY bright. I mean really bright! They might be too distracting to drivers next to me at night, as I haven't really tested it out yet. All I did was install them, but they light up the ground next to you, which you can't see in the pictures.
OSRAM Sylvania Ledriving Premium White LED Bulb 168 194 2825 6000K License | eBay