Out of curiosity, fessie, have you asked your usual grocery store whether they can stock it?
A few years ago, my wife wanted some vegetable ghee, but couldn’t find it anywhere in town. She asked an assistant manager at the local supermarket, and within a week they had it on the shelves (they continued stocking it, too, so presumably we weren’t the only ones who bought it).
Garam marsala seems like something any store ought to be able to provide, if they have reason to think there’s a demand.