buying tickets for baseball in september

Will i have problems buying the tickets when I'll be in Tokyo in September or I'll need to buy them in advance because it's need the end of the season?

I've only been to one game in August and I bought my ticket the day of the game from a machine in a convenience store. Some sections are more popular than others and sell out, but there were still plenty of seats left in certain areas of the dome. I imagine weekday/weekend and opposing team play a factor as well. I'm going to visit Japan again in September and plan to attend another baseball game. I will probably just check availability at the convenience store a few days before the game (or earlier if possible).