Amazon.com: HXJNLDC DC 3.7V 2100mAh 804050 li-ion Lithium Ion Polymer Battery Replacement for DIY 3.7-5V Electronic Product, Mobile Energy Storage Power Supply : Electronics
Lithium Ion Battery - 2Ah - PRT-13855 - SparkFun Electronics
Lithium Ion Polymer Battery with Short Cable - 3.7V 420mAh : ID 4236 : $6.95 : Adafruit Industries, Unique & fun DIY electronics and kits