Not everyone has that kind of money to be making use of a military grade vault to store their private keys. There are still many other ways that you will be able to do that without having to spend much money for it. The only important thing is that you never get to store it online where it is susceptible to hacking.

You can even make use of paper wallet, and after you have written your private keys on it you can then laminate it to keep it safe. And if you have a vault, you can keep the paper inside their safely. Some people also recommend cryptosteel  which is resistant to water and fire.  it is sold somewhere around $100.