Mine is the "Dollar Tree", big green letters in the name -- always thought it was Dollar Store... They seem to be individual franchises, I bet the one out by my mom's has fabric, they have a huge craft area. The closer one to me is bigger but they seem to have problems paying their suppliers and periodically they have been holes in the shelves or out of helium for the balloons and such. It was the smaller one also close to me that had the fabric, a little box in the craft area, the pieces were rolled up so it was on the top shelf below the hanging things. Got to admit, I don't always think to look but I do pretty often and have looked at many branches of the same chain.
Note on the label that the size is 20x21, so slightly wider than many fat quarters. Both were carefully cut a tiny bit larger (20.25x21.5) than that but turned out smaller, 19.5 x just barely 21 (selvedge) after the washing with the fraying and shrinkage. I only counted usable fabric, not the fringe left behind.
In terms of being off grain, it was no worse than many store cuts I've gotten, but that brings down the on-grain width to 16" on the polka dot.