You get them by reading the books being put out now by Fantasy Flight Press! You can find my reviews for the two books here and here. At the back of the books is a little sheet you can mail in with $2 or so for shipping to get cards for Arkham Horror. I sent in the forms from the back of Ghouls of Miskatonic and Dance of the Damned. Weeks passed and I forgot all about the forms, until last week an envelope arrived!
Now I just need to round up some vict, I mean friends to play with!