I've looked into this route previously but didn't have much luck as choices are a lot narrower than say iphone battery cases. But if you must, you may consider mipow juice cover and others. I don't remember off the top of my head but ilounge has reviewed other cases so if you dig there, I'm sure you will find some.
On a side note, there was one by ipega which interested me a little but upon reading amazon reviews, I was convinced that they were manipulated reviews as all of them consistently had only 1 or 2 lines followed by 5 stars w/ none of them being verified amazon purchases.