I can assure you that buying a ticket for a game at the gate is a 100% sure thing for the Swallows or Yokohama against any opponent, especially in early season. The same is true for Chiba and Saitama. I also you discourage you from going to the sterile Tokyo Dome to see the hated Giants.

You can check attendance at the NPB website to get an idea if Giants or Tigers actually sell out. Also, weekday games tend to be poorly attended because of the 6 pm start. The attendance numbers reflect tickets sold rather than people passing through the gates. I know this because I went to a Swallows game where the official attendance was 6000 and I assure you that there was around 1000 people in the stadium.

Regarding tickets in general (I would not bother for baseball), I found a place called Ticketport (a PIA outlet) that offers over-the-counter ticket sales to all events. I use the shop in the 109 building in Shibuya. It's on 2F in the tower facing the corner and you access from the outside. The key advantage here is that you can select specific seats in a seating section. If you buy online or at 7-11 then you cannot control the specific seat that you get. They also accept any credit card. The ticketing fee for two tickets is only 210 yen. (I think it's a flat fee but not sure.) For J-League (soccer) the tickets are typically discounted 500 yen if you buy in advance. I speak remedial Japanese, but the clerks at Ticketport also speak some very basic English unlike your typical convenience store clerk.

p.s. Note that the baseball teams also have box offices open during the day. For the Swallows and Giants this is not out of the way for most visitors.