Many buy from ebay or other sites, but these often don't have guarantees and you read stories all the time about people getting ripped off. They don't know that their tickets are fake until they show up a the stadium and can't get in.
I recommend either NFL ticket exchange (go to tickets on the Cowboys Website) or Stubhub. On Ticket Exchange you buy directly from the ticket holder, and when you purchase it it cancels their bar code and only yours is valid. Stubhub comes with a guarantee too.