If you are involved in the local ham radio community, towers are often available for free...............or almost free. You typically have to take them down, which is far from free.
Over the years I have had the opportunity to get many "free" towers.
It is fairly common for a local ham to die, and his widow contacts the local radio club to get rid of the tower. The widow wants it gone.
The problem I always had is that I never had anyone to help me get it down. I know a lot of hams, but when you start talking about taking down a tower, the number of people willing to help you dwindles.
Of course safety is a huge issue as well as having the equipment: climbing belt, tower jack, gin pole, some way to haul it away................................
Other than if someone dies, there are people moving that need to get rid of a tower in order to sell the house. And again, you usually find out about this because you are active in the local ham community. It is pretty obvious that if you are trying to sell a tower, this typically is a local thing. Not many people are willing to go take down a tower hundreds of miles away and then get it home.
Last year, a buddy of mine went back and forth with a guy for a month about a really nice 70' crankup tower. The guy was moving and needed to get rid of the tower. He was selling it; not giving it away free. But as time went on he became desperate to get rid of it. The price kept coming down. It started out that you had to take it down, later he offered to help take it down. As time moved on he offered to take it down himself and all you had to do was come and get it and haul it away. FWIW: my buddy never did get it because he didn't have any way to haul it It was pretty far away and he didn't have the means to haul it. That is how I became involved, he wanted me to go with him and use my truck.
As was mentioned, if you knock on the door, you may not get a tower but you might make some new friends. The last place I lived, I did that. I drove past a guy's house all the time that had a lot of really cool homebrew antennas. I knew enough about the subject that I knew these antennas may have been homebrew but they were pretty high end antennas. He had a four-square array on 40 meters as well as a 160m vertical. All made from stuff like irrigation pipe. One day I was driving by and the guy was outside in the driveway. I stopped and asked about the antennas and I got a couple hour lecture about them. The guy was retired from the defense industry. The homebrew antennas were very sophisticated and he explained the theory behind them and the test equipment he used to build them.
Was well worth stopping.
Another guy had FIVE, FULL SIZED verticals on 160 meters.
I would have never met either of those guys if I hadn't stopped and knocked.