Get to know the folks at your street and parks maintenance agencies. Storms create wood. they can tell you where they dump it.
Furniture factories often have discarded pieces due to knots, blemishes, etc. One here in town as about a skid load a week
Pallet and skid makers get larger pieces and need to cut to length. Some have piles of cut offs for free
Don't over look small saw mills. One near me makes 3.5 x 5 parts for pallets and trims them to 7 ft length. whatever is trimmed off is thrown in a free pile. Sometimes they make pallet wood from even elm, cherry ash, bass and walnut.
Tree guys
Keep an eye out for home owners cutting trees. Polite requests can get you large pieces of wood, but you need to be prepared to act swiftly and clean up. That may mean keeping a saw, tarp and broom in your vehicle
Drive around after a storm. You will see downed limbs, etc.
Get to know a heavy equipment operator. or a township permits official, They can direct you to lots being cleared for construction.
Some people list free wood on Craig's List and other internet community bulletin boards like Next Door, even facebook market place.
Join a turning club and learn to swap
always be polite, say the magic words even when turned down, maybe next time. give some small token of appreciation for the wood. Even a small ornament turned from their old maple tree, has some sentimental value for them and shows your appreciation.
Once word gets out, you will get calls and wood will accumulate.
Even free wood isn't always a good idea. Processing it and sealing ends, storing till dry, etc all take effort, and space. Learn to recognize the types of wood that are good for turning and those that are not. Learn the leaves, the bark, trees even have a certain shape that can be picked out a mile away. etc. A pile of freshly cut red oak, isn't nearly as good as a few pieces of black cherry or even a couple of pieces of Eastern Red Cedar. (A fellow in Maryland, 40 years ago, grabbed a piece out of a pile of scrub where a fence row was being bull dozed. He thought he had a piece of root of multiflora rose, he made a couple of pipes and sold them. Turns out it was a piece of poison ivy vine. and affected some of his customers .... adversely) Although a pen made from poison ivy wood and sealed well, would be quite a novelty ......" poison pen"
Keep in mind that our trees are under attack by a host of parasites, diseases and fungi There are strict rules some places about transporting wood from county to county. Spotted Lantern Fly, Gypsy Moths, Emerald Ash borer, Ambrosia beetles, Thousand canker disease, lethal yellowing, Chestnut blight, and many more. (I seem to be living where they are all converging. Recently lost all my ash trees, had a few black walnuts die and just recently had ambrosia beetles attack a Sassafrass. Spotted Lantern Fly has killed some trees just 9 miles north.