Almost all of the wood I use comes from a handful of arborists and tree surgeons within a twenty mile radius of where I live. Over a period of years I have developed relationships with them so that they know what I’m interested in and like to use.
The timber is almost always from domestic jobs where time onsite is at a premium, quite often I’ll get a text message with images of the felled tree and something next to it to give scale, either a boot or a person and a message saying something like: “If you want it you need to be here within the hour otherwise it will be cut up for firewood”.
To be fair, the arborists I deal with would rather see the wood used to make something rather than it either becoming firewood or chipped for biomass.
When those texts arrive I grab my chainsaw and safety gear, a bunch of sizing discs and spend the rest of the day cutting urn sized blocks and bring them back to the workshop.