Here's a great idea for a thank you that also gives you a reason to drop in on a regular basis to your referral partners.
Get an open glass container from Michaels, Hobby Lobby or WalMart and fill with candy. Tie a ribbon and attach your card. Drop it off and let your referral partner know that you'll keep it filled for them when you drop by.
Remember that referral partner's are NOT just Real Estate Agents. I've had financial planners, CPA's, and attorneys that regular sent me business. Take a look at where your business has come from over the past few months, you may even have some past clients that have sent their friends and family to you. Why not give them something for their desk?
Inexpensive, yet effective at keeping your name fresh in their minds.