It can be bought from gamersgate: http://www.gamersgate.com/DD-PATH/pathologic
Myself; I gave up. Couldn't stand playing it after a while. Now, over a year later I'm giving it another try.
There's few games that can change one's view of gaming as a medium, Pathologic is one of them. Spend a few hours in it, and it will be remembered. Few games leave such a lasting impression.