are you looking to buy or rent?
If buying, then you need to set up email alerts at places like uklandandfarms, zoopla, rightmove etc.
If renting, you need to figure out which agents in your area cover land rentals and farm sales. I've done this in the past simple by writing down the agent details whenever I pass a for sale / to let sign on a bit of agricultural land in my area.
Also, things like the farmers guardian used to be useful, or the farmers weekly if you're posh
HTH
Adam