Rakuten is a very reliable source that connects the English-speaking world with Japanese shops. You aren't going to find a better one online.
One thing about Kubota Slugger gloves
KSN = Nanshiki or Rubber baseball gloves (Between $200-$300US)
KSG = Hard ball (American) baseball gloves (Between $450-$600US)
KSG is superior quality but is twice the price. The overwhelming vast majority of us can play with the KSN nanshiki line.
My problem with Rakuten is even when its translated to english, certain parts are still in Japanese. Example is when it come time to select the shipping, all 3 choices are in Japanese. No idea what they say.