Best Buy offers the Rocketfish Power Shell with Rechargeable Backup Battery for Nintendo DSi
, model no. RF-GDS1128, for $1.99
with free shipping
. That's $18 off and tied with a Cyber Monday mention as the lowest total price we could find. Sales tax is added where applicable. It powers the DSi for up to 12 hours and comes with a USB charge cable and screwdriver.