Scalping is legal here, so there will be tickets for sale on every block within 2 square miles of the RCA dome. Last week they were selling Colts/Chiefs tickets well below face value just prior to game time. This week will probably be a little different but you can still easily buy them on the street.
If you're looking for a Patriots friendly hangout my dad (the only other Patriots fan I know in Indy) owns a bar on the same street as the RCA dome. He intends to start the pregaming around 2pm on Sunday.