Now I know this might not mean much to the people who read this blog but I'll keep trying to educate you in the ways of comics and graphic novels anyway. This is lifted straight out of Neil Gaimans blog:

If you want a free signed Sandman period Cerebus, you have to write to Dave Sim at Dave Sim Aardvark Vanaheim, Inc P.O. Box 1674 Station C Kitchener, Ontario, Canada N2G 4R2 Write him a letter. Tell him why you want him to send you a signed comic. Put enough postage on the envelope to take your letter to Canada. (That's a 60 cent letter or a 50 cent postcard from the US. $1.80 from Australia. Probably 47p from the UK. You get the idea.) He's getting letters from all over the world.