There are a few different ways of getting your hands on FreeSpace 2.
Good Old Games (GOG)
A new digital distribution service for classic games, including both FreeSpace titles, which launched in late 2008. GOG is selling both games for $5.99 (USD) with no digital rights management strings attached. Definitely the most highly-recommended means of obtaining the games at the present time.
GameTap allows premium users to play both FreeSpace 1 and FreeSpace 2. However, the service does not support mods of either game (meaning no FS2 Open) or online play.
There are still copies of FS2 floating around, but finding them is not always easy.
So where could you grab a copy?
- There are usually some up for sale on eBay.
- A Bargain Bin.
FreeSpace 2 has not been mass-produced for several years, so checking places that sell used software is your only recourse if you need to buy locally. The original FreeSpace has been out-of-print (so to speak) for even longer, but luckily for you you can play FS1 by getting The FreeSpace Port, but you'll need FS2 set up first.