I'm planning on using my x3i as a mp3 player in the delivery room next month and I'm concerned that the battery will not last as long as the labor.

So, I'm thinking about getting a spare to have charged and pull it out when it's necessary.
I know, first thought would be to bring the power cord, but we've been told a variety of times, no power devices. So, a battery is the only option.
So, to the point...any recommendations on good places to get a replacement, the bigger the better for this, I'm not too concerned with it fitting in the leather case I have either. Just need to calming effects for the wife!
Any and all help appreciated! Thanks.