From FreeSpace Wiki
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 FreeSpace 2 for $9.99 (USD) and the original FreeSpace for $5.99 with no digital rights management strings attached. Definitely the most highly-recommended means of obtaining the games at the present time.
FreeSpace 2 was available on Steam in June 2014.
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.