$15, really? I'd have expected at least $30.
I had done some research on batteries (or, I should say I Googled it a bit), and I found those 1600mAh batteries. I also found various things on ebay. I really don't trust cheap chinese electronics, especially when said electronics can heat up to a few hundred degrees in a fraction of a second.
That said, I'd love it if Nintendo (or some [genuine] 3rd-party producer) made upgrade batteries of 1600mAh or more. They're coming out with technologies to increase the batteries' capacities tenfold. I think the cathode is generally the limiting factor of Li-ion batteries, these days, but I'm sure there'll be a breakthrough in just a couple years.
(Alternatively, I've been looking at combining multiple batteries. If you get two batteries and connect the positive to positive and the negative to negative, you'll get the same voltage but twice the capacity. I'm not sure exactly how this will work with li-ion, but I believe you just need to add some form of regulation to the circuit. Good to research if you're inclined.)