You can use any 3.2 or 4 ohm speaker you can find that will fit.
Look for an electronic parts dealer near you.
There aren't many left, but you might get lucky
and find one that has been in business long enough to have older parts in stock.
Check with "surplus" electronics dealers online, too.
A few I can name readily are ...
Herbach & Rademan
And --- how do you KNOW that the speaker is bad?
Have you connected another one to test the amplifier?
I see in your pix that the unit has a tube amplifier.
Have you tested the tubes in the unit,
or substituted them with known-good tubes?
There is a good chance that the problem is related to old capacitors and resistors.
Have you checked those parts?
Lastly, small output transformers are likely to fail.
Have you tested yours for continuity, particularly the primary winding?
[With the unit disconnected from power, pull the output tube
and use an ohmmeter on the red and blue wires.]