You've Got Mail!
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!
Inside were more envelopes. Sinister...
The Ally card is from Ghouls of Miskatonic and the Mythos card is from Dance of the Damned
Oliver is one of the protagonists in Ghouls of the Miskatonic. He's made a damn good ally in the game too!
I don't know why you'd ever want to play with this card! The game is hard enough as it is...
Now I just need to round up some vict, I mean friends to play with!