Our trip begins on Wednesday and man are we excited!

Is there any way to purchase Tube fare (OysterCard?) ahead of time? I realize that I haven't left us much time, but perhaps there is a sales affiliate here that sells fare?

Or is it easy enough to obtain upon arrival to Paddington Station?

I'm thinking something akin to the $7 NYC "FunPass....*"

thanks, Lewis

* Remember when it was $4?