I got my "Atheist Money" stamp from evolvefish.com... along with a pirate fish window decal for my car, so I can be all Pastafarian without being TOO blatant about it. ;-)
You can get custom stamps made pretty cheaply. I got another stamp (unrelated to atheism) and ink supplies from rubberstampchamp.com and you can make stamps saying whatever you want there for very little money.
Welcome to Atheist Nexus----the world of reasoning, rational sense, and good friends.
Founder of the Nexus discussion group
"ORIGINS: Universe, Earth, Life, Humankind, Religion, Darwin..."
We seek to understand the mysteries of the universe and astronomy, and how life and humans and religion and everything began. There's a lot to talk and think about.