This is something that will vary throughout different areas. In general, usually the best places are local estate agencies in the area that you are looking. In Scotland, you will find that the country is divided up into areas and these are covered by centres such as the HSPC (The Highland Solicitors Property Centre). There are variations on this theme for other areas such as Tayside, Edinburgh etc. Some of the self build magazines also have plot directories in them. If all else fails, keep your ears open and ask around. You might be lucky and hear about some land before it goes on the open market (its a great excuse for a pint in the bar that you want to become your local).