I bought my tickets before I left and printed them out. No problems other than I was the only Giant's fan sitting in the Softbank Hawks section over the left field fence. The site didn't mention that.. However, nobody bothered me though. (Japan is very friendly) Security was paranoid for me and my wife though.
Here's the link to the Giant's website to buy them. (I don't know about other teams, but I got this link from Tripadvisor's forums.)
https://gticket.e-tix.jp/en/ticket_pc_en.php