I went with a Japanese friend to a store called Nojima, which seemed to be the Best Buy equivalent for Japan. There was an entire wall of prepaid SIM cards, and there was a promotion for two 6-month SIM cards for 12,000 yen. I wouldn't have been able to find it had I not been with a Japanese native.

I got a 1 month SIM card for 2,200 yen ($22) which still works out to far less than what I pay Verizon. The network name on the top of my iPhone said "docomo".