If you are running a GNU/Linux system, you almost certainly already have access to Bash. It is installed by default on almost every GNU/Linux computer system.
On such systems, it is very often the default login shell for users. This means that when a new user logs in for the first time, it's the first interactive program that runs, and it starts up to wait for command input from the user.
On some systems, such as Debian GNU/Linux, Bash will be the default login shell for non-system users, usually human beings rather than system processes, but a different shell, such as the POSIX shell or Bourne shell, will be used for system accounts.
Bash can be installed on other Unix-like systems as well, such as on FreeBSD, NetBSD, OpenBSD, or proprietary versions of Unix. Even though Bash has such a strong history with GNU/Linux systems, administrators of these other systems often end up installing it, because it is so popular and many users will expect to be able to use it as their shell. It usually has to be installed as a separate package, and is not part of the default installation.
You can also build Bash from source on most Unix-like systems with access to a C compiler. Doing this is outside the scope of this book, and you should use your system's packages or ports system if you can. The Bash source code is available at https://www.gnu.org/software/bash/.