HKJ did a review on some Amazon NiMH batts that ran 2000~ MAh if I recall, and the price was not too bad
P.S. a single 3.2v LiFePo4 AA and a dummy AA works just fine, in fact a LiION 14500 and dummy work fine too, even at full charge 1vdc shouldn’t be too much to damage a 3vdc circuit…but your choice on the risk…