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
Briefly, all of the answers submitted are worthwhile. I just had a case where a former client went off to do just this on his own, then ran into a lot of difficulties after closing that could have been avoided. Sure you can do it yourself, but if your goal is to have a good outcome, call on any of the folks who responded or an experienced agent in your area to represent you. Not sure what other agents are willing to do, but I am always glad to write letters on behalf of my buyer clients and typically my mailings will cover 25-100 property owners depending upon the buyer's wants and needs.
However, before doing that you and your agent should discuss your goals and objectives to see if what you want can be accomplished in your area. Though I know a number of excellent agents in your area, I am not familiar with the Hampton Falls market for raw land. I can tell you that a lot of owners of residential building parcels in other parts of the state have not grasped the fact that there are some excellent values in existing homes today, that the cost of construction services and materials are through the roof and that the prices of raw land have to reflect these realities in order to compete. If you need a below-market land price to make your finances work, you may be taking a long walk on a short pier. But for an agent who is really active in land, the incremental effort to assist you is small and you may be able to find a good value.
Your agent should be able to get you to "yes" faster with a potential seller and may have other motivations for getting their name out there amongst land owners in your area for future business. As others point out, someone who does this all the time and is focused on real estate can save you a lot of start-up headaches and help avoid pitfalls during or after closing.
Either way you choose to go, good luck
Chuck Braxton, REALTOR GRI
I agree with the last two answers. You can utilize all of those sources for potentially spotting a piece of land.
Might I also suggest talking to a few developers and construction companies in the area.
I would expect the first source would be the real estate agent, one working in and familiar with the area would be the ultimate source though.
If I can be of any assistance, please let me know and good luck with your search!
Or, hire a good real estate agent.