Shouldn't be difficult. T-mobile and AT&T will be your largest carriers of prepaid SIMs. No need to register your name and personal info. All the U.S. cities where you'll fly into from HKG or Narita will have these carriers. I'm not sure you'll have the option to find the SIM cards in your arrival airport, though. You'll likely have to wait until you are into town.
T-mobile prepaid link.