Where can I get a threaded wooden/metal rod for an adjustable piano stool?
If you have a router, you can use the Beall Wood Threader. A kit to thread one-inch dowels is US$85. They have kits to do smaller sizes or multi-size kits. Parts are included to do both inside and outside threading.



You can find a hand tool on eBay.



Here's a source for ready-to-use threaded dowels in half-inch and three-quarter-inch diameters up to three feet in length.



Here is a source for metal hardware: Lee Valley Tools.



Additional information can be found by searching for "threaded dowel" or "piano stool threaded".