Ebay offers DeWalt 20V MAX 2Ah Li-Ion Compact Battery for $29 with free shipping.

Product Description: 
The DCB203 20-Volt Lithium-Ion Battery pack is compatible with the complete line of DEWALT 20-Volt MAX tools and accessories. This battery is backed by a DEWALT 3 Year Limited Warranty for added peace of mind.  

• Compact lithium-ion pack provides up to 33% longer run-time than standard 20-Volt MAX battery packs
• 3-LED fuel gauge system allows immediate feedback on state of charge
• No memory and virtually no self-discharge for maximum productivity and less downtime
• Lightweight design and quick 45-minute charge time
• 20-Volt lithium-ion battery pack
• Compatible with the complete line of DEWALT 20-Volt MAX line of tools and accessories