I picked up one of these:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=170186517763

Came with a USB charger cable, 2x BP-5L compatible batteries, and a desktop charger (the desktop charger has no power adapter, so it requires the power adapter that comes with the N800 or the included USB adapter to work). The Nokia E61 uses the same power plug and batteries as the N800, and everything seems to work just fine. At $14 including shipping it was a pretty good deal.

I realize this is an old thread, but I bought the item linked below and can report that it works fine on the N800. I know that at $14.99, it isn't as cheap as the item previously linked, but it has the advantage of being readily available in the US at Circuit City:

http://www.circuitcity.com/ssm/Kensi...oductDetail.do

I came home from work yesterday without my AC-4 and that was my excuse to run out and pick this thing up. Works fine.