The following are leasing companies that I have worked with on securing vertical real-estate over the years. This is not a total list of tower companies. If you provide co-location services to Wireless ISPs and want to be included please reach out to me. Donations motivate me to update these lists. Ones with a Star next to them are WISP friendly from our dealings.
American Tower
https://www.americantower.com/
Clearview Tower
http://clearviewtower.squarespace.com/
Crown Castle
https://www.crowncastle.com/
Heartland Tower
http://www.heartlandtower.com/
Insite Wireless
https://insitewireless.com/
Melody Wireless
http://www.melodywireless.com/
MidAmerica Towers
https://midamericatowers.com/
Nexus Towers
https://nexustowers.com/
Subcarrier Communications
https://www.subcarrier.com/
Tillman Infrastructure
https://www.tillmaninfrastructure.com/
TowerCO
https://www.towerco.com/TowerSearch
Towersites.com
https://tower-sites.com/
Tower Ventures
https://towerventures.com/
Vertical Bridge
http://www.verticalbridge.com/