http://www.highlandhorn.com/natural-shanks/9.htm
On holiday just now, but to get you going i'd just buy from here when he is back.
Re-your own shanks
You have the chance of splitting the shanks by putting them in a boiler room to try and dry out.
To be honest any shank will do, but Hazel, Blackthorn, Holly and Ash tend to prove to be the best.