T-Mobile has announced a few official Extended Range LTE expansions in recent weeks, like in El Paso and the Jersey Shore. And there have been a few unofficial band 12 LTE sightings, too:

Cupertino, Calif.

Palo Alto, Calif.

Blackfoot, Idaho

Rexburg, Idaho

Buena Vista, Colo.

Hartsel, Colo.

Bergen County, N.J.

Sources: Reddit (1), (2)

Thanks, tazzman101, Joseph, and Adam!