Battery Problems

First it depends on your phone, be aware that some phones have their batteries on the phone , but even that way you'll be able to get batteries if you find the right person. If you send that to the store it will cost a lot specially on that kind of phones that have their batteries incorporated. 

You can find batteries on stores but in the time you have to decide if worth paying like imagine 45% of the original price of the phone to fix the battery that will get worse in a few month or buy a new phone if it fits you and you think it's a better choise for you.