Some people say some of the European eDGe sellers have them and will ship to the US (see the "Devices" tab on th enTourage web site). Other than that, you might find the odd battery still in stock at smaller electronics retailers that carry the eDGe, or possibly college/university computer stores. You can also wait for one to appear for sale on eBay.

There's no indication any enTourage products are being manufactured anymore, so inventories will dwindle to zero and after that continue as resale of used items.