I purchased an extended external battery unit that will charge pretty much anything that can plug into a USB port. New Trent is the manufacturer and was going to post a link to a new model of the one I got but the forum spam check won't let me.
I have been very pleased with it. I just got a Droid 4 today and will use it with it as well. I would suggest going out to Amazon or Newegg and seeing what your options are. Read the reviews and figure out which one fits your needs and your budget.