I've seen b-mobile SIMs at Bic Camera but I'd imagine any of the larger denki retailers would carry them. I also mentioned Amazon.co.jp in my last post. Click "In English" in the upper right corner of their home page and put "b-mobile" in the search box. BTW, you'll need an unlocked phone which you can also buy on Amazon or in various shops around Akihabara (if you don't already have one, that is).
Freespot is one of the public WiFi services. I gather some of the APs are actually free but many may not be. Google for "tokyo public wifi" and poke through the various forum posts. I saw a number of recommendations but didn't really follow up on any of them. Contrary to what some folks believe, I have other stuff to do besides doing someone else's research ;-).
BTW, at first I didn't believe that Sofbank's WiFi service required a contract but their web page suggests they do. That's kinda odd considering all the Sofbank WiFi stickers I see on store fronts around town. But Sofbank always did strike me as an odd provider.
Oh... one other suggestion. If you don't mind hamburgers, my kids have been able to get a free WiFi connection for their Nintendo devices at several McDonalds around town. I've never tried it from my tablet before but the next time we're there, I'll give it a shot and see what happens.
Scanning down the list of SSIDs my tablet seems to have picked up in its travels, I see 7SPOT (most likely 7-and-I/Denny's), FREESPOT, Wi2 (and Wi2premium which may be a paid provider -- suggesting Wi2 might be free), CG-Guest (who knows, maybe some company whose building I got too close to ;-), and, of course, FON and FON_FREE_INTERNET. None of them are marked in my list as being secured by WEP/WPA keys so those might be good search keywords to start with. I also recall seeing something about BBMobile, which is b-mobile's WiFi service. And some of the ISPs in Japan (like GOL) offer their own service that probably piggy-backs on one of the other WiFi provider's APs. You can sign up for a GOL account for about 500 yen per month and use a non-Japanese address. Asahi-Net (which I used for a while before I got cable) used to also offer a public WiFi add-on but, as I recall, they didn't have a "lite" package for those who don't really need the physical connection.
Google is your friend...