Unfortunately can't get them easily here in NZ, one "reputable" store sourced one from the Nokia distributor and the consumer price was like $73 (that's what around US$55)!
I found a USB to 2.5mm cable on TradeMe (like eBay but with a much higher pentration in NZ) for only NZ$12 and it works a charm.
Boxwave has a USB datacable for the N800 that has a y-split to mini USB B and ac port of the N800.. haven't tried it but seems like the best solution out there.. I wonder what the pin outs would be from the usb cable to wire your own charger from a standard cable usb cable..
It is basically a retractable usb cable with interchangeable tips. the cable was $4.95 and each tip is about a buck. I have about 5 tips and it works well for all of my devices. It is exactly the same as the nokia product, but even better.
Proporta also has some other cool products that can be useful for the ITs. Check them out.