Ordered 2500mAh from Adafruit off Amazon. https://www.amazon.com/gp/product/B01NAX9XYG
—Updated Battery Times—
Running CaveStory:
GS Battery (1200 mAh) = 2h 55m
3DS XL Battery (1750 mAh) = 3h 40m
Adafruit 328 (2500 mAh) = 5h 23m
Note: The settings I am running with are backlight brightness at 2 with wifi on. When I checked amp draw, those were the two biggest factors. No other settings had a significant change for draw that I could tell. With Wifi Off the times may be higher, and With brighter backlight may be lower, etc, etc.