Lithium Ion Cylindrical Battery - 3.7v 2200mAh : ID 1781 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Amazon.com: 1Pcs 18650 3.7v 4400mAh 1S2P Li ion Rechargeable Battery Pack with 2.0Pin JST-PH Connector 16.28Wh/4.4A, Blue, 6.6 x 1.8 cm (19.28Wh/4.4A) : Health & Household