You're referring to the 2015 13" MBP? They use a blade SSD which has a connector that is proprietary to Apple. These SSD's are available from people who have removed them other Macs on places like eBay. Apple does not sell them separately.
Yes, but not internally. There's no space, and the connection that Apple provides will not accept a standard SSD.
If you want to upgrade, it has to be an Apple device, pulled from another Mac.
You can find them on eBay.
Or, alternatively - add another SSD externally with a USB case (needs to have support for UASP), or a (more-expensive) Thunderbolt enclosure.