You could try and buy a copy of the Estates Gazette from larger branches of WH Smith and look at the small ads. Or drive around and see if there are "plot for sale" or similar signs by the side of the road (more likely along a motorway or dual carriageway though).
Depends on the type of land you are looking for. Most residential estate agents don't do land, but I know of several agents who do, in various areas of the UK - what part of the country are you looking at? And are you looking for building plots, agricultural land or woodland etc?