Where can I buy some musto?
If indeed you mean grape must, that is unfiltered, pressed grape juice, I can't help right now.
But, come fall (sometime in September), there's plenty of places that spring up: Italian "Uva de vino" or 'wine grape' stands. They sell various types of wine grapes (alicante, zinfandel, muscato, etc). Some will press the grapes for you, putting the must into 5 gallon pails. Others will pre-press the grapes of their choice and you can simply buy them in the 5 gallon pails.
I'm pretty sure they pop up at the Jean Talon market.
Also in Lasalle.
I'm confident you can find them in any neighborhood with a large Italian population.