Libraries are particularly good places to find these titles and they tend to last a long time, especially the later editions in CD format. Larger bookshops too often have dedicated audiobook sections. However all the newest versions have been published as download only.
With the 2017 rerelease of all of the authors novels, Paul Tyreman has been working his way through the canon, reading each novel in unabridged format. As far as I can tell the only one left to read now is The Snow Tiger, a trickier title than most as it is in the third-person.
These new versions are available from Audible, a subsidiary of Amazon, via their subscription service.
Previously titles were available on cassette-tape from Soundings Audiobooks, Chivers Press, Hamlyn’s Books on Tape and Listen for Pleasure, but these are now harder to find. Second-hand and charity shops are now our best bet for actual, physical media.
A few cassettes are still available at Amazon UK, from third-party sellers and eBay is much the same, but the prices seem high.