are you sure you need a new battery? See this excerpt from the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support
"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge."
In other words, if it is below 80% maximum capacity, it is below normal performance. However, per the article, Apple would only consider it unusual (and warrantable) if that drop to 80% happened in less than a year after you receive it. Ergo, storage loss as you use it is expected and acceptable within the parameters presented in the support article. Incidentally, that 20% drop/year equates to almost a 2% drop per month so 1-2% per month is not unusual. You can still use a device with a lower percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80%.
Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support
For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)
For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support
