I don't know of any stations that play nothing but garage rock... no terrestrial broadcast stations, anyway. Lots of stations have specific shows that feature garage and trashy punk/rock'n'roll, though.
My favorite Internet station, though, is WFMU's Rock 'n' Soul Ichiban. I have a wifi radio (which I HIGHLY recommend if you have wireless Internet at your home) and have that station on almost all the time. I can seriously listen to it 24/7. Of course, you can also program it to stream podcast feeds, too, and I have the GaragePunk Podcasts on it, too.