Hi,
Search online for a ''BA700 battery.''
This battery is used in a Sony Ericsson Xperia Ray (ST18i)
Here’s a link to just one [https://www.ebay.com/itm/New-Genuine-BA700-battery-For-Sony-Xperia-Neo-Xperia-Pro-Xperia-Ray-MT11i-ST18i/162931944544?hash=item25ef802c60:g:-pwAAOSwkVZZ4wd-|supplier].
It is shown only to give an idea of the cost of the battery. There are other suppliers online that may suit you better.