If I understand you right he is refering to a tree. If you are interested in chasing a ring Osage would get my vote. If you want to make a white wood bow then Ash or Hickory would both be good choices. Any of the woods mentioned would work great though.
Ask for as long a piece as is practical, this gives you some grace room in case of bad knots or splitting ends.
The greater the diameter the more staves you can get of course and it will give you a less rounded back. I have made bows out of 3" diameter trees, bigger than that is better though.
Unless you want a character bow, ask for something straight, not twisted and with as few limbs as possible.
Have fun and tell us how it turned out!