Any one of the home supply stores, like HD or Lowes has it. Also any paint store worth a darn should have it. If you want a small quantity to mess around with go to a hobby shop and look in the art supplies department. Michaels or Hobby Lobby. You'll find little bottles of it there.
Gunshops and Sporting Goods have a Boiled Linseed Oil Finish for Gunstocks. It has a Drying Agent and a Hardner in it. Look at the abuse a Gunstock takes... I bought some at K-Mart...