I see the Nokia-brand ones in just about every mobile phoneshop I've been into. I don't see the point in buying one, as stocking up on Li-ion batteries is not very smart: they just don't keep well. The best thing to do is wait until your current battery dies and then get a fresh one.

Maybe a tip: I recently bought a spare generic wallplug charger that came with a "reduction plug" (from the old Nokia large tip to the new small one that's on the 770 -- the tip is available separately from Nokia, but it costs almost as much as I paid for the charger). I subsequently used this reduction plug on one of those emergency mobile chargers that I had lying around (the ones you stick 4 AAA batteries in and plug into your mobile when there's no wallplug around) and the 770 happily chirped it was charging! If I did my maths right, I can get at least one full charge from 4 alkaline AAA's, which means I can run my 770 for as long as I can stuff AAA's in my backpack and sod to all the eco-solar crap.