They have kashk (and everything else Persian) at Persia Foods.
They have 4 locations:
1469 Marine Drive, West Vancouver
118 w 15, North Vancouver
6437 Main St, Vancouver
3827 w Broadway, Vancouver
The traditional kashk is dried whey in the shape of small balls. These balls are boiled in water to reconstitute the kashk.
Today kashk is normally sold in bottled form. It is a thick paste which you add water to and boil. You will want to use a few tablespoons of the paste with a cup or so of water and boil it until it is about the consistency of a stew (some people like theirs thinner; I prefer mine thicker)