There are a lot of theories about this, and threads you can read. My suggestion would be to start by expressing appreciation, and seeing if there is any interest in selling. A kind of 'Hey, I really love those old kettles', and see where the conversation takes you. Much more friendly and less threatening than pushing in there and trying to negotiate right away. And if they aren't interested in selling, you might leave your number with them for the future.
Another common tactic is to offer a trade with a more modern kettle. In fact, many of the Grillfellas perpetually keep a cleaned OTS that they picked up cheap or free for 'trade bait'. That's often mighty appealing to someone who is looking at a greasy old kettle every weekend.