If it is the LED brake light that is out (not functioning)...then the fix may be a bit more complicated than the inner brake light bulb.
If it's the inner brake bulb that is out...then as listed in the owners manual...just type
21w h21w bulb into a google/internet search box to find vendors selling that bulb online. Or stop into your local Walmart/Target/AutoZone/Advanced Auto/Pep Boys/O'Reilly's/etc and go to the automotive bulb section to find a
21w H21W light bulb.