I'm assuming people aren't busting out their axes, felling their own lumber, then seasoning the wood in a controlled environment just in order to lathe out some sweet discs. However, I guess I could be wrong.
Replace axe with chainsaw and you've got it. My buddy Clay does quite a bit of harvesting for the purposes of turning, I often give him leads, or give him a hand. The wood dries in his shop, or turns it wet and dries it in the kiln or in the basement for a year. There is also quite a bit of trade on ebay, someone else does the work of harvesting and drying the wood and he pays what the going rate is for a slab of maple burl. He is always on the look for sources of wood - any major land development project that's going down, he's there seeing if he can dig through the piles of wood before they're chipped or milled.