how to write a proposal or letter of inent to purchase industrial land that is not for sale?

Get the contact information from the state division of corporations. Send a letter sayiing that their property fits the specifications of your buyer client, who is serious and qualified. Ask them if they would be willing to engage in conversations leading to a potential transaction and find out the price points. Follow up with a phone call.

Call them on the phone.
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Gregory Garver - Commercial Real Estate Broker
Broker License# 01716531
(415)968-6065
greg@nnnbrokersusa.com
http://www.gregorygarver.com

Mohanie....
The owner of your land likely knows the corporation and other information regarding the ownership of the adjoining parcel. If not, you may at least find basic information from the department of corporations in your state. (A title search of public records may also be helpful).
I have found that a phone call to the company asking who handles their excess property inventory, or whom is their real estate asset manger, often provides the proper person's name. After receiving this name and proper contact information, you can either direct an LOI to the proper party; or better yet talk with him or her on the telephone to discuss availability and deal points.
If they are interested in selling it is then simply a matter of beginning the process toward a satisfied seller and a pleased buyer!
Onward and upward.... Rob Baird
Onward and upward....