Websites: Here, Zillow, Realty websites (Remax, Coldwell, etc.), Craigslist, Google searches.
Don't underestimate local marketing. I love to drive the neighborhoods I'm interested to find the FSBO and expired listings. Yard signs and billboards may work well since they are in a different context than the online world where you have to filter so much info in a small place.
It sounds like you are marketing a product that is not a traditional mortgage. If that is true, you may want to try first line marketing to contact buyers before they start looking for a mortgage. With the Realtors pushing preapprovals so heavily, I'd target buyers before they found the house they want. Once I speek to a realtor they ususally all have a lender they push really hard.