I can only speak for getting tickets while here in the US. I don't think it will be a problem, depending on the specific game, if you purchase early... perhaps very early.
Generally speaking, tickets go on sale before the first pre-season game. This would be the time to buy imo. They should be easy to come by at that time.
FYI, depending on the schedule, the Cowboys may only play the Texans during the pre-season... They may not play during the regular season... so pay attention and plan accordingly.
At the end of the day, if you plan early and purchase early, you should be able to get tickets to any game you wish.
All the best, if I can help feel free to PM me.
ETA: don't forget to account for the cost of parking, it can be very high depending on how close to the stadium you want to park. Or, the cost of a taxi/Uber to the stadium, they jack up the prices on game day.