I just searched the internet, and I found some Dutch sites that sell posters with Severus on it (like this one link )
I figure that if you search the net, you will be able to find a site that sells them in your country (but I haven't found a poster with just Severus on it)
Good luck!

EDIT: I showed this link to a friend of mine. He ordered the poster for me. Jay :)

Yeah.. Posters with Severus on it are very very hard to find.
That's why most fans print their own. I know I have. I had even made a life size stand up, it all depends upon the resolution, the higher it is the better the print out. Hope this helps some :)