I'm not so concerned at finding a SIM and cutting it down to size if necessary. Plenty of flyertalks carry a micro sim cutter through security. It looks kind of like a larger hole punch. As for availability, even Softbank sim rental carries micro sims and I'd consider Japan to be one of the tougher countries to get a sim for. Plenty of carriers have an iPad sim or data plan.

But how does one get around the carriers which require sms to activate certain data subscriptions without carrying a spare phone? Some carriers still require you to sms a code to a number to get a data plan instead of signing up on a website. Is there a sms spoof app you can sideload that will spoof the sim's phone number?