How to write company's introduction letter to a company as a dealer?

Letters of introduction are just not used much anymore, and 99% get tossed.

Provide a glossy color brochure of prices, terms, and a few words of praise from other clients.

Even that is passe. Now you work on tags and keywords and ad clicks online.
If you want, print postcards to companies with your website on it, but even that is a waste of paper, printing, and postage these days.