6BX6's were used in a lot of early Australian TV's (1956-1960) prior to the use of 6EJ6 & 6EH6 in the video IF & sound IF stages.
Try the following links
http://www.tubedepot.com/nos-6bx6.html
http://store.triodestore.com/6bx6ef80.html
http://www.vacuumtube.net/nos-6bx6.html
http://www.dc-daylight.ltd.uk/Valve-...-for-sale.html
Good luck, It would be great to see this old panasonic working again...