Where can I find good ground sumac?
As with almost anything you can purchase ground, you can also purchase sumac berries whole:



You may be able to buy them whole locally, you can certainly buy them whole online, as that picture is from Amazon. Spices lose their flavor quickly once ground, it's almost always better to grind your own.

If you don't have a spice grinder, I recommend checking out local thrift stores for a used, rotary style coffee grinder. Even new, they're very inexpensive, and then you'll always be able to grind your own spices.