Not available at conbini anymore. At least not according to someone I asked here in Japan. Prepaid (purchasable at a Softbank shop, some electronics stores, or Costco if you're near one) is easier and cheaper if your Japanese friend is willing (remember, they're responsible if you lose the SIM, someone else picks it up, and uses it for bad things).
Alternatively, if you're mostly calling out and don't expect much incoming, the best option is a Chinese SIM. These roam in Japan for a smaller deposit than Softbank wants (typically 600RMB/7506 yen, all credit for you to use) and they're cheaper per minute, too (in Japanese currency, local calls are 25 yen/minute on a China Mobile card and 23 yen/minute on a China Unicom card). They also need ID registration too, but in my experience, most street-vendor SIMs have random people's names and IDs attached to them that count for this purpose (and tokyo_dom, yes, I look very silly taking a rental keitai that charges 10 times as much when roaming from China is so cheap- let's just say that I found out the "most" part while frantically trying to activate international roaming at the airport an hour before the flight to Narita departed). That's what I did last time I came to Japan. The only annoyance was that people who called me were paying international rates (thank goodness for cheap international phone cards). And you have to find one, probably before you leave for Japan.