The current issue of Woodshop News features an article about Denny Zimmerman who harvests burls. I, too, am a "burl hound.”
Everywhere I go I am always looking at whatever trees are within sight, trying to spot burls. I often have to explain to my wife as she sees my head spin around that I'm looking at burls, not girls! That might be fodder for a psychiatrist but we'll let that go for another time. The point is that if looks could kill, there would be a lot of trees lying by the side of the road.
Buying burls is ridiculously expensive, especially "clean" burls, which almost always get slurped up by the veneer mills. I use burls a lot for accent pieces and in my chessboards. Finding them is a much better way to acquire them than having to buy them.
I got on the email lists of every city within reach that has any kind of tree ordinances and I watch them for proposed tree removals. These agendas are often accompanied by photos of the trees in question so it's easy to see which of them have burls. I have acquired a lot of really nice burls in this manner. Most of the tree cutters will cut the burls out for you right there on the spot if you get there when they are taking the tree down. They mostly seem glad to have someone want to use some of the wood and it's that much less they have to stuff into the shredder.
There is also a local wood dump in my town and I have mined a number of really nice burls out of it. Usually people take the wood for firewood and if the piles get too big, the city comes in and chips it all up. I usually stop by there at least once a week.