YellowLetters.com is a site where you can create direct marketing created for investors and brokers. They provide mailing lists, yellow letters and postcards, bandit postcards, etc. to fit all your needs. Pricing starts as low as .40 per postcard including postage.
We recently sent 1,700 mailers, over half to the wrong zip codes, but still we got 40 phone calls and ended up buying three houses from it!
An example of what a yellow letter might look like is below:
There are other sites like YellowLettersComplete.com and Click2Mail.com that also create personalized mail for you.