Oat flour is an inherently gluten-free flour* made from whole oats ground into a fine powder. You can replace up to 1/3 of wheat flour in a recipe.
*You may notice some oat flour products are labeled gluten-free, and some are not. For example, Bob's Red Mill sells two versions of oat flour. One is labeled whole grain oat flour and another whole-grain gluten-free oat flour.
The reason for the difference is that the product labeled "gluten-free" is produced in a portion of the facility with equipment that does not share lines with their other wheat production products. Bob's Red Mill says to use their whole-grain gluten-free flour as a 1:1 replacement for all-purpose flour in a recipe.
If you have some rolled oats in your pantry then you can whip up this oat flour at home (not just an alternative):
You can purchase oat flour in many grocery stores. Trader Joe's and Walmart carry oat flour. If you're shopping on line there are many options for oat flour at Amazon.com: Oat Flour
The great thing about oats is that they are very soft and grind quite quickly and uniformly in my small coffee grinder. I placed two tablespoons of the oat flour in the mill, and within seconds I had perfectly ground powder. If you need more than a few tablespoons, then you will want to use a blender or a food processor to mill your oats.
2 tablespoons of oats in a coffee mill
Finely-ground oat flour