I've done a pick-axe handle bow, and it made a shooter....but for board bows, a hickory backing is the most economical, and easiest to obtain and work with backing. I've used them on a number of wood combos, and can consistently build bows in the 70 plus pound range that perform as good or better than any stave bows that I've built.
I've found that hickory backed maple is an easy to get and work combo and will produce great bows in heavy weights. Also purpleheart, Ipe, yew, osage, and many other woods. But the maple is readily available as well, and works great in combination with hickory.
Bamboo is also a great backing, and performs perhaps even better than hickory, but is harder to work and much more expensive, especially on a "learning" budget.
Making your own hick bckings is as easy as sawing them out of a straight grained board on a table saw. They glue up well with Titebond 2 and 3....
No better way, IMO, to learn tillering techniques and to get the "feel" of it all before moving on to more expensive materials.