Inexpensive AA battery charger:
www.amazon.com/gp/aw/d/B073W6GH9Y?psc=1&ref=yo_pop_mb_pd_t1
Looks like the prices have gone up, but they have plenty of panels to choose from, and still under $70. These are the two I have:
www.amazon.com/dp/B00OQ0CAW6/ref=cm_sw_r_cp_apa_vkSJAbDCKCGZJ
And with a built in battery:
https://www.amazon.com/gp/aw/d/B077PXSHJF?psc=1&ref=yo_pop_mb_pd_t1
I also got this battery backup that I use almost daily, and I really like it: