I bought my Firefly direct from NRD at www.nrdfirefly.com. It is an expensive gadget, but it has favorable reviews including one by Jason Odell here on Nikonians several years ago. I had the first version and found it a bit cumbersome to use because it required two hands. The new version is designed to be used one handed which I like much better.
I cannot say that the Firefly cleans my sensor better than just a plain Rocket Blower, but I like the concept and like to think that it does.