Russo's always has them, though they can be a trial to locate. They tend to keep them separate from the other citrus, I suspect so people won't accidentally mix them up with the tangerines and the like.
They always have them at Somerville Market Basket, and did this a.m. Sour oranges, that is. They are right by the plantains (always reminds me to make mojo!) near the ice cream freezers.