We’ve updated the QuadVol page. Now, the part list is online. For German customers, Reichelt Elektronik is usually a good source for standard components. However, both ICs needed for this project are not available at Reichelt. The volume control IC is often available on eBay. However, there is a risk that you get a fake IC. Therefore I would recommend to buy the parts from a well-known supplier, like Digikey, Mouse or Farnell.