It is availbale, almost, everywhere but be careful with the version, different Linux diatros are at different versions, the last time I used OSX it was stuck on a very old version, and I expect the different BSD OSes to run fairly new versions.
BSDs don't ship Bash as part of the base system - you have to install it from packages or ports. And that one is the most recent that the maintainer bothered to package. E.g. FreeBSD is on 4.4.23 right now, which actually appears to be newer than e.g. Debian unstable.
