There's not much one can say about section 131 that hasn't already been said. You can never go wrong with seats behind home plate. It's a big section with around 40 rows so it's not touch to score a ticket in section 131 for a Rockies game, period. I would never recommend dealing with Ticket Scalpers but Stubhub seems to always have tickets behind home plate for every game at varying prices.
If you really want to do it big, buy the 'Coors Clubhouse' seats, those sit down in front behind home plate and are closer to the catcher than the pitcher is.

A few tips you should be aware of when venturing into Coors Field; you can bring your own food and bottled water into the park. Not sure what their policy is on coolers/backpacks. You can also re-enter the ballpark after exiting by having your hand a ticket stamped by the stadium staff. Ask one of the ushers where you should go to begin that process.