>>175189You go and buy boards because it easiest for beginners to start properly making stickbows from a board.
Making proper bows from trees is not the easiest thing for a beginner to do. You have to find an acceptable tree with straight grain and free of knots for a beginner. You then have to cut it down seal the ends, split it into staves and then let them dry which can take a few months or more.
Mimosa wood is too brittle for a good bow unless you use all heartwood, and then it's still not great.