In college I bought a pair of Polk Monitor 5 speakers for $200. They were probably built in 1981, which now makes them old enough to vote but not quite old enough to be elected U.S. President. They were in my living room until a few years ago, and I still listen to them in my basement when I'm exercising. Or at least I did until my cat puked in my amplifier (below) and blew them up.
Polk sold this woofer, the MW6500, to technicians for many years to make repairs. Then they made a near-equivalent replacement, the MW6502, for even longer - people were still buying them in 2008. But after 30 years it's not reasonable to expect a company to continue to offer replacement parts. They recommended I contact the good folks at Madisound for a substitute. We settled on the house brand Madisound 6102 woofer (the left pair below) as the only one that would fit the highly constrained front panel of the speaker. It sure does fit - even the screw holes are in the same places.