Browns seem to always be scarce when you're ready with the greens so what I did this year was make a big pile of browns from my spring cleanup. It's mostly left over leaves from last year's leaf mowing that didn't breakdown over the winter, twigs, dead grass, acorn caps, stuff like that. A pretty good mix of things actually.
This fall I'll screen some of the pile and store it in totes for use in my soil mixes and spread the rest of the pile across the garden and flowering plants, and will start a new pile with the fall garden/yard clean-up.
Circle of life and all that.