That would be very cool. Have you tried eBay?
Most of the autographs I have bought have been through people who can authenticate their products.
There actually is a magazine specializing in autograph and photos of famous people.
One of the weirdest autographs I got was one by Emerson. Actually it was a letter of recommendation of his nephew to Harvard.
I snapped it up for under 100 pounds.
The reason I wanted it so bad, was because the letter was addressed to then Harvard divinity professor Henry Ware, one of my ancestors.