What if you created a web site that explained the game and gave cool examples, then print up that web address on sheets of clear address labels, stick those to regular cards you don't want, and drop those?
Because on a spiritual level everything speaks. Everything is governed by the law of Seed, Time, and Harvest. Remember, you reap what you sow. We've thought of that, but it could send mixed signals. It'll look cheap, dinky, unorganized, tacky, and something done out of desperation.
Having actual printed cards would have much more of an impact and a real message. It'll present itself more uniform and with a purpose.