Monoprice has a variety of good options available and reasonable prices. You might be able to find one that suits your specific needs there. They often even go on sale for a bit cheaper if you check their once a day.
If you don't plan on using the X3 for more than 8-10 hours a day, then you probably won't have to worry about getting an external battery.
I find that I use my portable players about 2-3 hours a day tops. So normally I can go about 3 days before charging. Sometimes I forget that it's low and forget to charge, in those cases it could be useful to have an external battery around to tide me over for the day.