All Galaxy S3s should use the same size battery.
Check AliExpress. They have reviews on which are best.
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20171130183550&SearchText=galaxy+s3+battery
This is the top seller on Amazon.
https://www.amazon.com/RAVPower-Replacement-EB-L1G6LLU-GT-I9300-GT-I9305/dp/B00KNJJPSG/ref=sr_1_1_sspa?s=wireless&ie=UTF8&qid=1512095826&sr=1-1-spons&keywords=galaxy+s3+battery&psc=1