Got any 2-way radio shops in your area? If so then check with them to find out who they use as a antenna/tower contractor. But the cost of buying and taking down a tower to erect at a different location might be higher than buying a new one and installing it. But if you are going to do the work yourself then check your local Trader Magazines, usually free, all kinds of stuff advertised for sale.
I will tell you right now that most used towers are used and for sale for a reason. Having been in the cellular construction work for some 18 years I have seen my share of good and bad towers. There have been tower that want to rent space that are lucky they are still standing. One tower that we were going to go on, we sent a tower crew there to inspect it. They came back and told us the had condemned it. About 300 feet up on a 550 foot tower, one of the climbing rungs came off the tower in the climber's hand. The tower had rusted from the inside out. The hollow tube construction had allowed moisture to do it's damage over the years and now it was going to be cut down.
Not that I am saying you can't find a good used tower, but the reason most people don't want them any more is that they can't pass inspection and load requirements. The tower specs have changed over the years and an old tower can't even hold the antenna load it was designed for and meet the current standards.
Used 70' self-supporting? You might luck out and get a decent one, but like Jim said above, many have structural problems. One just came down last week north of Bowling Green, about a 250-300 footer. It had been up for many years, and they knocked it down into the farmers field and cut it up. If it had been any good, space would have been rented on it.