I've just started subscribing to
Radio Detective Story Hour.
Typically it's a few minutes of introductory background by the host, followed by an archived OTR ("Old Time Radio") detective show, vintage advertising and all. Each episode features an episode from a different series (except for some additional episodes from a few favourite series), so it's a good survey of the various sub-genres.
If you think you might like to hear more episodes from a particular series, it's very likely that there are dozens more in the
OTR section of archive.org. The only catch is that those aren't podcast feeds. Either download individual episodes mp3 files manually, or download them all as one big zip file.
Of course archive.org also has OTR comedy series, suspense series, dramas, soap operas, variety shows, newscasts, big band specials, westerns, anthology SciFi, space opera episodic series, and so on. Generally, the older ones are noisier and scratchier, but there's buried treasure everywhere.