I use the hard drive category on pcpartpicker.com, sorted by price per GB. You can then filter by seller and total size if you have other constraints.

https://pcpartpicker.com/products/internal-hard-drive/#sort=...

Also be sure to check external drives. Sometimes it's cheaper to buy one of those and then discard the enclosure.

https://pcpartpicker.com/products/external-hard-drive/#sort=...