Crepe myrtles get this white and purple stuff on the leaves every year around August. The trees seem to survive, but it doesn't look healthy, particularly because the leaves get all curled up.

A fungus of some kind?

Looks like the beginning of powdery mildew.
The best way to prevent powdery mildew on plants is to use a preventative spray formulated with sulfur.

I guess I would find it in a garden store?

Ivan Mann wrote: I guess I would find it in a garden store?

Yes, but make sure this is really the problem!
I just guessed...