Phone companies, banks, major retailers, and major industries are good places to ask about this. A few companies have even assigned staff to work with non profits on a part time basis. Most health care and international aid organizations appeal to people's pity. We are in the wonderful position of appealing to people's optimism. Make sure that those representing you are visibly enthusiastic.
Enthusiasm is infectious.
Choose well spoken and well informed representatives. Make sure that they understand business conventions and dress. Every freenet will have a somewhat different policy on business relations and commercial activities on the net. The more you can demonstrate that your free-net meets the needs of the business community, the easier it is to fundraise. I have written a paper describing what I believe are the major advantages businesses may get from the existence of freenets in their area.