The cheapest way I’ve found other than stealing them from the Internet is by spending a load of money. If you buy 24 credits from Audible for something like £110,this gives you the cheapest cost per book you can then supplement these with the Daily Deals and the frequent sales.
In your situation just try and get one of the many audible trials either one credit free when you sign up or 3 months membership at a discount. Good bonus of Audible is that if u you don’t like the book they refund your credit /cash
We get quite a few audio books from the library. they have thousands so we just order the ones we want and they get them for us from other libraries/a central store. I’ve brought a few from eBay too. Buy them, listen to them, sell them again usually for what I brought them for.