its not available, there is no linux port,
you can try to get it running through wine.
Last i checked though, they had some code in it to stop winex from running it.
It is definately possible to get it running in wine, but difficult
check
http://linux-gamers.org , they have howtos and stuff.
they also have a cvswinex available.
I only ever play it at lans, so what I do is I get one of my mates to share his folders and, download the whole thing, straigth of his hard drive. then i run it with cvswinex, simple solution.
Im not a 'cs' player myself, i just like shooting cs lamers from time to time :p