This has been around for 2 years now, but you may not have heard of this previously; if you purchase the deluxe edition of Brian Wilson's Smile from amazon.com UK there's a 1 in 4 chance of a piece of paper with his signature enclosed.
My wife bought this for me last Christmas and we found the paper. It's a replica of the album cover. Sure enough, written in Sharpie was Brian's signature. If I can get one after two years of it being out, you might be able to as well. But be warned, it's quite pricey. Here's the link;
http://www.amazon.co.uk/dp/B00035SDDI/?tag=imwan-21Good luck. David