I grind my most of my own GF flours. I make oat flour in my high speed blender from old fashioned oatmeal, so that is cheap. I found brown rice grinds better in the grain attachment that came with a juicer, so again, cheap.
Unless you don't have a high speed blender. I got one of those years ago, and love it, even though it was $400. The juicer I bought used from a friend, just because it was only $25, a high quality brand, and the grinder was "something that came with it." My blender can't grind the rice quite as well as the juicer. Even so, you can feel a bit of grittiness in the flour, but it disappears when baked.
I feed my SDS (sourdough starter) half oat, half rice flour. The few times I've made actual sourdough bread, I've used some sorghum and tapioca flours with. Still experimenting to get more rise, but the flavor is awesome!