It's a little shady, because the song (off their upcoming Halcyon Digest, out September 28) is supposed to be reserved for people who printed out Deerhunter posters and hung them up in their hometown. But here's how even non-hanger-uppers can get the MP3, courtesy Pitchfork:
If you go to this website and enter the password "tapereel," you'll find a picture of Deerhunter with some hidden links in it. One of those links (the one on the reel-to-reel tape player) will allow you to download the new, invitingly glimmery new single "Revival" ... and the non-album B-side "Primitive 3D." (It takes a while to download, but be patient.)"
So, whatever you feel comfortable with. (Or just pick it up here.)