My dad's Zenith - been in the family since new

[Electronic M]

Most audio output types are hunted by audiophools who have more money than sense.....They buy them up voraciously and the price climbs skyward.
You might find them cheaper somewhere, but don't count on it.

To expand on dieseljeep's point: When the audio coupling caps get leaky and bias the audio grids positive (it may still sound fine this way) the cathode current in those tubes skyrocket which rapidly uses up the cathode.....Prematurely killing your expensive audio tubes....So unless you want to spend that $40 in output tubes again soon after the new ones are installed, you REALLY SHOULD recap the audio section.