Option #1: Negotiate a separate installment plan for the down payment. Sometimes the seller will allow you to pay the down payment on a monthly basis.
Option #2: Trade something other than cash. This could include land, a car, a boat, jewelry or valuable collectibles. Find out what they want and need. Maybe you have, or can get, just what they are looking for. You could also trade services such as carpentry, auto mechanics, painting, dental work and other services that you can do for the seller over time.
Option #3: Find an investment partner. Look for an investment partner who will put up some or all of the cash in an equity-sharing partnership. You make the monthly payments and the two of you split the eventual resale profits.
Option #4: Get owner financing or a land contract. Another option is to have the seller act as the bank. You make your payments, including interest, directly to the seller. Then after usually 3 to 5 years you make a lump sum payment to the seller. During this time, you should have enough equity to qualify for a standard bank loan.
Option #5: See if you qualify for any down payment assistance programs locally (though I think they are for owner occupied properties).
Option #6: Hit up mom and dad for an early inheritance payment.