If you search on Amazon.com you'll see a few listings for aftermarket replacement Kindle 2 batteries! It's not hugely hard to do if you're experienced at disassembling electronics. But if that's not something you're comfortable with, I'd opt for an ereader with a user replaceable battery like the nook. The IREX also has an easily replacable battery but they've declared bankruptcy so it's probably not a good time to buy one of their products.
But e-ink ebook readers have very long battery life, especially if you turn off wireless when you don't need it. And given how infrequently they require charging, the battery should be good for several years before it needs replacing.