Here's a pretty cheap copy on Amazon
it has been 20$ on amazon for a while now(years I'm talking) https://www.amazon.com/Demons-Souls/dp/B002AB7TX8/ref=sr_1_1?ie=UTF8&qid=1479706049&sr=8-1&keywords=demons+souls
This one? Yeah i'll probably get it too. I don't plan to get a ps5 until at least 4-5 years, since i got my ps4 in 2019.
Check ebay for used -- best I saw was $11 but with $6 shipping, you're better off getting it NEW from Amazon for $17. If you have Amazon Prime (or you have an a friend or relative that would buy it for you that does, shipping is free. If prices are the same or slightly higher for new, I still go new every time. Best of luck in your hunt.
http://www.amazon.com/Demons-Souls-Playstation-3/dp/B002AB7TX8