Any 6x9 speaker will work, as long as it is 8-10 ohm. Most modern car stereo crap is now 4 ohm, but all household audio is still 8-10 ohm. so you can check a home stereo supplier.
Unless you are wanting concourse correctness
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Beaumontguru
MY BEAUMONT HAS 4 STUDDED TIRES AND 2 BLOCKHEATERS......AND LOTS OF OIL UNDERNEATH. The other one has a longer roof.
Part # 7282782 was superceded to 1221838 by July 1970 (maybe before) as per a July 1970 Pontiac catalogue I had a look in. 1221838 was also listed in the July 1969 Beaumont catalogue as the service rear speaker for 1965-1966 Beaumonts.
the speaker shown in the ebay listing looks exactly the same as the one in my 70 2+2 hardtop. GM used that same basic speaker for many years. Later ones are basically the same, but the electrical connector is different. Later ones can be used, especially if you have the insulating/protector box under it, as it won't be seen.