Prepaid LTE SIM by So-net offers 3 basic plans (1G, 2.2G, 3G), and their SIM card is available to purchase at various locations throughout Japan such as airports and major stations.
Almost all of Dospara stores, a large computer-related chain that has 20 branches all over Japan, sell Prepaid LTE SIM cards. And if you have a plan of shopping on PC-related items, it might be a good choice.
You can top up easily from your user page from 3 additional plans.
For a more information about So-net - Prepaid SIM Card Plans