Walmart discounts their patterns 50%, so if you don't have a Joann's available, that's the next best deal. They carry Simplicity and McCAlls.
Vogue has a couple of patterns that are "time period" perfect for Molly, Emily, and Kit, but those can be more difficult to follow.
Don't buy any of the vintage patterns from online websites, because a lot of those don't have clear markings. The seam allowance isn't marked, and even darts and tucks are just marked with holes and dots. The instructions aren't easy to follow, either, although, depending on the age of your grandmother, those may be the type of patterns she grew up with. I know that a lot of my mothers' patterns were that type.