Check with Richard Ritter as I remember reading that he can make various springs of all sorts from stock in his machine shop. With some size dimensions and tension criteria I am sure that he could kick out something pretty quick to meet your criteria.
They should be pretty easy to make. You can get the spring steel from McMaster Carr(www.mcmaster.com). It is tough stuff to cut, though. You could probably cut it with a corner of a file or use a cut off wheel on a Dremel tool. The corners can be rounded with a file or a grinding wheel. You can make the bends by clamping the steel in a vice and bending it. If you don't have a vice, clamp the part between two blocks of wood and bend it. I've made prototypes at work this way and it works fairly well.
Take a lok at the thread with PF frames there's a view of the back of one. They used clips used for retaining screens or storm windows in a storm door. $4-5pkg of 4 at the hardware store. Not the prettiest in town but functional.