Do you have a particular book in mind? You could try Amazon and eBay from time to time. Amazon sell both new and second hand books. I recently bought a copy of 'Bella' by Jilly Cooper, which was first published in 1976. It's been printed up several times over the years with different covers. I went for a one from the early 1980's
Perhaps one day the British Library will scan the covers from their vast collection and publish them online. It will, of course, be a 'work in progress'