You can buy the game from any on-line store but I would Highly Recommend you buy it directly from STEAM

Here is the Steam app id or link where you could buy it from steam
http://store.steampowered.com/app/10/‎

Though it is highly recommended you buy the game or you can download a Cracked one. So I will Recommend you to download a cracked Counter Strike 1.6 from CSWarzone. It is updated after 2 days after the steam version is updated. So here is the link: Click here to go to CSWarzone

Note: Though you will be able to play with the Cracked game with no issue but you won't be able to enter Steam servers. So it is Highly recommended that you buy the game and support the Developers of such an Epic Game