I personally think red oak board from Lowes or some such is perhaps the best starting place, the wood is dry and cheap,
gives you a chance to get an idea of the tillering process.
Take a look at Jawge's website lots of good info on it.
http://mysite.verizon.net/georgeandjoni/archer.htmlSmall saplings can yield great bows and large logs can be less than good, depends on the type of wood and the design.
Some woods you use the heart wood only, most white woods like Hickory, ash, elm etc. you peel the barl off and thats the back of your bow.
I suggest you do some on line reading and if possible invest in a few books. Paul Comstocks book The Bent Stick is a affordable book that will get you headed in the
right direction, the 4 book Traditional Bowyers Bible series is a must have also and of course a subscription to Primitive Archer magazine is also well worth the money.
All are available from this site
http://www.horsefeathersranch.com/