Scalpers are on 5th and 6th street between Hennepin and 1st Ave for the most part. As you found out, tonights game was in pretty high demand, surprisingly but with Rays, weekend, give away, fireworks, and bombers it was fairly packed beyond just the game.
Stubhub and Ticket King are options, you can always get tix from the Twins themselves as they don't sell out. I know someone that was GIVEN 2 tickets (lower level, 3rd base line) when approached booth at Target Field after the game started (1st inning was underway).