Ebay really is your best bet. Bartertown here on AK may see a few knobs periodically (you have to be a subscriber to access it), but there is no place more likely to have the knobs you need at any given time, nor in the needed quantity. Keep in mind that you can search for related models: 5050, 6060, 7070 ,8080, 9090 (and DB variants on the latter two). QRX-8001 and 9001 share several of the same knobs as well, so that gives you two more potential parts donors.