Amazon.com: 2 Pcs 3.7V Rechargeable Batteries 1/2AA Lithium ion Battery icr14250 300mAh (Flat Top, 0.57 * 0.98inch, Shorter Than AA and AAA : Health & Household
3.7V 18650 flat top cells for battery packs
Lithium Ion Polymer Battery - 3.7v 100mAh : ID 1570 : $5.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Amazon.com: FPVERA 3.7V 1200mAh Li-ion Battery 2PCS 3.7Volt Rechargeable Batteries with USB Charger Cable SM-2P Plug for RC Car Drift Monster Truck Off-Road Vehicle Remote Control Car High-Speed Climbing Racing : Toys
Lithium Ion Rechargeable Battery - 18650 - 3200 mAh - 3.7 V – GuardDogSecurity