OWC (Aura Pro X) and Transcend (JetDrive 820) now make SSD's that will fit in the 2015 MBP, however, both have a max of 1TB and the OWC (and likely the Transcend as well) require High Sierra. I haven't seen in-depth reviews of either of these products yet.
There are adapters that allow you to fit standard PCIe drives (including 2TB SSD's) in the 2015 MBP, however, likely because of the high-performance nature of these SSD's, they can be a bit iffy. If you're do go this route, I would suggest getting a SSD which uses MLC vs. TLC NAND chips for heat and power consumption reasons. There's a long thread on using standard PCIe drives in the 2013-2015 MBP's:
https://forums.macrumors.com/threads/upgrading-2013-2014-macbook-pro-ssd-to-m-2-nvme.2034976/
Another option you have is using SD flash drives in combination with the OWC/Transcend SSD.