Hm, that's a good question. I would be less worried about the frequency than I would the differences in electricity and outlets. As I learned the hard way, a lot of basic electronics in the US won't survive the voltage found in the EU (where I lived) and Japan.
Actually, in Japan, there's a great children's store called Akachan Honpo, where my wife got her baby monitor when we stayed in Japan for a few months (our girl was about 14 months). I highly recommend the store for any baby goods you need to get. I am not sure where Misawa is, but if it's near a major city, you should be in luck.
Best of luck in any case.