This product was mentioned in
with an average of
You can purchase pocket wi-fi devices from Amazon for sure (search for 3G/LTE hotspots and you'll get a listing for stuff like this: [link]).
If you visit japan on a regular basis you might want to check out Bic Camera or Yodobashi camera. I'm pretty sure i've seen them on sale there. Go to the flagships in Akihabara because they offer tax-free shopping (I assume you that you are not a japanese resident hence you are renting a SIM) and could also probably find English-speaking staff to help you.
Do you have access to a mifi router? This is what I use, works like a charm. Doesn't need to be the top-line model, but the ability to have your own network with you is a luxury you don't want to miss after a while.