As cassettes became defunct technology around a quarter of a century ago, the best place to seek them out is probably eBay:
https://www.ebay.co.uk/b/Comedy-and-Spoken-Word-Cassettes/176983/bn_557222
Almost all charity shops ceased selling audio cassettes well over a decade ago but some still make an exception for spoken word cassettes, so it might be worth keeping an eye out there for anything that interests you.
Thanks Chris, ebay seems to be a good way forward. Also looking around on the internet it seems that cassettes are far from over, now they are being collected & so I read are making a comeback.