We get our chips from a local arborist who we use for all of our tree work. So, we get our own chips when they're here (maybe every 2-3 years), as well as anytime that I call.
They usually show up within the week, or the next week. I have asked for just one load, and I have also asked for 100 cubic yards (they made 3-4 trips over a two week span).
There is a service called Chip Drop, which is a sort of matchmaking service for arborists and gardeners. I have not used it since we already have a good thing going with our arborist...
Chip Drop