i was in American India Cash & Carry on Pleasant St, in Malden a couple of weeks ago and saw that they sell pre-made dosa batter. Went back last Friday and picked some up to try it out. I don't recall seeing it for sale at other markets - has anyone seen it elsewhere?
My dosa-making skills are limited but I was pretty happy with the results. A pint of batter cost $3.50 and I got 6-7 dosas, making them in a 12" skillet. The batter is unsalted; I asked the woman working there about it and she said that some people prefer much less salt, so they sell the batter without adding any. (I ended up using about 2 teaspoons of salt for the whole pint, which seems like a lot to me but it needed it). The batter tasted even better today (Tuesday) after a few more days fermenting in the fridge.
They also sell frozen coconut and cilantro chutneys, $1.99 for a four-pack. We only used one container of each for all of the dosa (dosai?) that we made. So overall it was a pretty cheap way to get a dosa fix :)