Aircheck collecting
DecaloMania is the main club for radio aircheck collectors.
Aircheck archives
The “original” radio aircheck site is ReelRadio.
There’s nearly 5,000 airchecks spanning decades in the searchable manfrommars.com database.
Airchecks also exist at the Good ol’ Days Olt-Time Radio site.
One search term for finding old broadcasts is “old-time radio” (OTR). RadioLovers.com is the most popular OTR site out there.