Nintendo no longer has replacement batteries, Is ebay a reliable source for oem ones? I know there's a lot of fake oem batteries out there for other handhelds like the psp :
For Black & Decker 20v 3.5 Ah 6.0 Ah Lithium ion Battery/Charger LBXR2520 LBXR20 | eBay
DEWALT DCB201 20V MAX 1.5 Ah Lithium-Ion Battery for sale online | eBay
Genuine LI-ION BATTERY 10.95V 2100MAH 23W For Jett RZ1020 SCUD RAZOR READ NEW | eBay
Li-ion 18650 Batteries
9V 9 Volt Li-ion USB Rechargeable Batteries 5400mWh Lithium Ion Battery Pack Lot | eBay
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