Just buy them on eBay. Why does it matter where they come from? Again, four of them have to die before it's no longer worth it. It's extremely unlikely you'd be that unlucky.
Personally I have 15 drives in my NAS, all of them were bought used and they've been running 24/7 for 4+ years without issue. Originally I expected to lose at least one per year but they just keep chugging along. All of them have at least 40k power on hours, with the oldest 3TB ones having over 80k (9+ years)
I use unRAID so if/when one does die it's as simple as pulling out the dead one, popping in a new one, and letting it rebuild itself.