HP ProLiant DL380 G7 LFF - Is every SAS HDD compatible?

HPE HDDs will have custom firmware so "ordinary" HDDs even from OE HDD vendor will work just fine (and you'll even keep your warranty, please see my link below) but you won't have special features like health monitoring / LED indication working.

https://support.hpe.com/hpsc/doc/public/display?docId=emr_na-lpn11980

Question: Will adding third-party memory or disk drives void the warranty?

Answer: Adding third-party memory or disk drives does not void the warranty. However, there are some implications when you need technical support. For example, when the memory used may relate to the problem you are experiencing, you will be asked to remove the third-party memory to confirm the problem still exists before a service provider is dispatched. HP does not cover on-site visits caused by third-party products, whether internal or external to the HP Netserver system processor unit.

There's no good reason not to use HP disks here.

They're available and very inexpensive on eBay.