When you are in the "gathering phase", you start looking for bow (and arrow) wood EVERYWHERE. You will start evaluating the grain of furniture, hand rails, fence posts, etc. One way to get osage and black locust, if you are in an area where they grow, is to look at Craigslist or a local farm auction for fence posts. (Sometimes ERC also.) All of those have rot- resistant heartwood, so you don't have to worry as much about how they were treated. Posts are often 8 foot sections, so you can trim the ends to get past checking.