It is possible to run (practically) any version of Postgres on (almost) any unix-like system -- Simply grab the source code from the Postgres web site and compile it (detailed instructions here).
If you're feeling ambitious you can follow the packaging instructions for your operating system as well and install your downgraded database as a package, but you can skip that step just as easily with few ill effects.
It is probably best to compile/install the old version in a unique path (
/usr/local/postgres82, etc.) to ensure that your OS won't stomp on it.
Note that when going this route you need to be aware of when underlying libraries change (which could break your database server if Postgres is linked against the library, necessitating a re-compile), and you will probably need to install your own startup script in a way appropriate to your operating system.