Just did a bit of pazarenje yesterday at the Market Basket in Chelsea, recommended by a friend for their selection of Balkan products. It is one of those huge supermarkets in which you feel a sense of accomplishment for having got from one side to the other (a local family-run chain, though, no cold dead hand of the Sainsbury Corporation here). In fact there is a small section in the eastern part of the "Ethnic" aisle, but it has many of the basic necessities: vanilla sugar, kiseli kupus from Macedonia in a jar, Bosnian mineral water. The meat section also has the right kind of suho meso for sarme and pasulj. But the real wealth of the place is in products from Central and South America. The selection is not just limited to the mediocre products of the Goya company, but includes all the sizes of dried corn a person needs, several brands of masa harina, and frozen and refrigerated pupusas and arepas. Not necessarily a wildly lucky find, but good enough to be worth some return visits. Be ready for crowds.
Update: A friend of East Ethnia observes that not everybody lives at a convenient distance from Chelsea, but that people in this category may be able to use the services of this online distributor. I have never ordered from them, and so cannot recommend one way or another, but they do seem to have a nice set of offerings. Though I would be inclined to take a pass on the frozen ćevapi by mail, myself.