Amazingly, it's getting harder and harder to find Doom2 in the stores these days.
You can still get Doom2 directly from ID Software's vendor, if you can't find it in a store. The link to buy Doom2 is:
Yes, it is true that the Doom2 source code was released by ID software. This was done so that Doom enthusiasts could make new Doom2 based source ports, and does not mean that the commercial Doom2 product is now degraded to freeware status. It is still commercial software. And since it IS still the best single-player and deathmatch game in existance, it doesen't hurt to buy it if you want it.