All great answers below. Another option that I thought of could be to look for current plots of land in the area that may be able to be subdivided. Look up the owners in the public records and attempt to contact them to see if they might be interested in selling some of their land. The success rate will probably be low, but it is worth a try. Also, you could look at expired listings.
I checked for you and there are three land listings in Hampton Falls right now. I'm not sure if you have already looked at these or not. Here is the info (sorry about the formatting).
MLS Address Town State MLS List Date List $ Acres Zoning LO LA Photo Indicator DOM
AC 4002840 L 2 Hardy Lane Hampton Falls NH 06/02/2010 $159,900 4.27 res/agr 2082 12853 1 228
AC 4002845 L 5 Hardy Lane Hampton Falls NH 06/02/2010 $159,900 2.02 res/agr 2082 12853 1 228
AC 2761911 L Lot 6 Kensington Road Hampton Falls NH 02/13/2009 $160,000 2.01 Res/Agr 2519 20680 3 702... more