#1- Ikea Ladda/Panasonic Eneloop.
They're likely the same thing, made in the same factory in Japan. They last the longest in my experience. The Ikea ones are much cheaper, if you have one nearby. Make sure they're in stock before going, at least at my store the AA ones are hit and miss, the AAA ones are always available.
#2- run it off an external battery pack.
I use a 20,000mAh pack sometimes. Lasts for days, especially if you're holding on a single frequency and battery saver is "on" in the scanner settings. This works best if you're stationary without access to power, I.E. sitting in your yard. I personally don't do this on the go, too cumbersome and worry about wear and tear on the connection.
That said, battery life with any batteries seems atrocious with this radio when compared to the 3 battery models like the 436hp.