Hi,
Here’s the [https://download.lenovo.com/consumer/mobiles_pub/lenovo_z_p_series_hmm_v1.0.pdf|hardware maintenance manual] for the laptop, taken from this [https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/ideapad-p-series-laptops/ideapad-p500-notebook/documentation/doc_userguide|webpage].
Go to p.44 to view the necessary pre-requisite steps and then the procedure to remove / replace the battery.
On p.91 Item #4 you will find the part numbers for the batteries. Search online using the appropriate part number ''only'' in the search box of your browser to find suppliers of the part. Usually the part number can be found on the battery itself if you wish to verify which one it is.