Do you want an original ~100 year old copy, or do you just want to read it?
It is still in print, if Amazon carries it, you can probably order it at your local book store.
Amazon.com: Scouting for Boys: A Handbook for Instruction in Good Citizenship (Oxford World's Classics) (9780192805478): Robert Baden-Powell, Elleke Boehmer: Books
There are about 10 copies on ebay for under $10.