If you're buying lists and mailing yellow post cards, that can get pretty expensive.

Just an idea, but what about business cards stating what you are looking for, your contact info and a short statement about a referral fee.  

I would get business cards and door hangers to start with.

Send a card that looks like it came from a family member.

I even have business cards that say so.

In any case, this often provides me with a free "out" card even when I waived everything else. 

@Brent Kiger I've been using rentigo for a while now and am really happy - @Cornelius Dudley is right - the tenants pay a service fee for using credit card or ACH and I'm assuming that's where they make their money from. 

Have you heard of drop cards?

Card?

I have no debt (car paid off, no credit card debt, no student loan debt) and pay no mortgage on my current home, so the $3k is unburdened cash.