The best lead you can get is one from an existing customer. Many customers would be willing to give referrals but don't think to do it unless they're asked. Make it worth their while. Cash incentives, and discounts are often used to reward customers for referrals which turn into sales.
1234, Main Street
Boston, MA 02123
09 April, 2005
<Recipient Address Goes Here>
We value our <customers> and hope you feel the same about us. There is no greater compliment than to receive a referral from satisfied <customers> and we would like to thank you.
Our <referral program> was designed to reward our <customers> for passing on the names of acquaintances who might <manner of use>. For each referral which turns into a sale, we will <reward>. <Optional additional terms.>
We hope to hear from you soon. Thanks!
encl: <List of enclosed items goes here>