I remember having a conversation with Sally Burnett on this topic. She gets whole sycamore trunks from a local tree surgeon. Her work is mostly large open hollow forms (vases), with diameters up to 16 inches (or possibly more), so she needs a lot of large blanks.
If memory serves, she pays about £300-400 for a 2ft sycamore tree, all small branches removed, delivered to her workshop and cut into pieces as instructed by her.
Mind you, this is for a very specific purpose. If you are not that picky, do what I have done: I am the webmaster of the Black Country Woodturners, and the club gets contacted numerous times each year by people having to bring a tree down and they simply want to see the wood go to a better use than being burned. Sp more often not it's free (but you'll have to go and cut it up and transport it and store it all yourself).