Often this is the most difficult part of the whole process, as dreams of a perfect plot are halted by the reality of limited availability and high prices. Yet perseverance often has it rewards. Below are the basics of what you should do:

Contact estate agents local to where you wish to build. Often they will have a website and will put you on their mailing list.

Get to know the area. Speaking to local people will give you inside knowledge of what land may be available and at what price. 

Be brave. Identifying a plot and asking the landowner if they are willing to sell can pay off. The worst they can say is no. 

Check the Internet. A dedicated website such as Buildstore Plotsearch can help.