Once you add your NFL blazer to your cart, you'll see a pop up appear asking you what pant size to pick. You can choose to pick a specific size, or not pick any pants at all. Hell, just go pantless. Definitely at your own risk though.
If you want to buy em after the fact, just hit the search bar (upper right corner for PC if you're on mobile in the upper left ) and type in the specific color of pants you need. If we have em in stock they'll pop right up.