Is oat flour homemade or it needs to be purchased?

I agree with Smaug’s answer but it’s also very easy to make your own if you already have old fashioned oats. Just blend the oats to a flour consistency.

Ms. Medrich is a pretty meticulous recipe writer- she'd tell you if you were expected to make your own, though if you know how no reason not to. Bob's Red Mill has a couple of kinds of Oat Flour. Bob's Red Mill has everything.