Any 5v USB 1 amp or higher will work fine.
I say 1 amp because YIHI says the charger in the
device has a 1 amp charge rate.
You may get away with a .5 amp (slower charge) but it may get hot. I never tried it.
A higher amp one say 2 amp wont hurt any because the device will only pull the power it needs.
Any best buy or radio shack should have one. Even most cell phone chargers will work fine.
Just read the MICROSCOPIC writing on it first and all the specs are there.