I won’t disclose where I store mine.

But I’d recommend to:

• Not backup your PW manager’s database + key file in the same location
  (That would decrease security, x1 data breach would allow them to easily brute force your PW DB since they’ll have the key)
• Not go with a PW manager that does not allow you to choose a location where you desire to backup to (Seen plenty of mainstream PW managers getting data breached by now, so going with a cloud, which is not solely used for PW managers, has an advantage imo, since they tend to be less targeted by hackers)

I’ve been happily using KeeWeb + Keepass2Android for years now:

• github.com/keeweb/keeweb
• github.com/PhilippC/keepass2android