I always steer people who want to buy tickets to the Steelers ticket exchange.
Right now there's nothing listed but they usually have a wide range of seats. Season tix holders just got their tickets last week so I think the site will be up soon.
If you make travel plans for a game later in the season be mindful if it occurs during a flex week, The time of the game may change during those weeks. Also you want to factor in weather and travel delays the the latter part of the season. (ie snow)
Good luck! I hope you get seats.