There are always staves on the online auction site, just got a real nice hackberry stave there myself.
I have not worked with it before, but I hear hackberry is some good whitewood.
I have used hickory wheelbarrow handles (basically a 5ft 2x2) to make shorter bows, those are around 10 bucks at Lowe's, Home Depot, etc. You could also use axe handles, if you want to do some handle splicing, those are hickory.
Mike