Amazon.com: MCE Technologies 2TB SSD for MacBook Pro 2015 15-inch Only: PCIe-Based 4 Lane (x4) NVMe 8.0GT/s SSD Flash Storage Upgrade - 2900MB/s Read, 2100MB/s Write, Requires macOS 10.13.x (High Sierra) or
Amazon.com: JESOT NGFF M.2 NVMe SSD Convert Adapter Card for Upgrade MacBook Air 2013 2014 2015 2016 2017 and Mac Pro Retina 2013 2014 2015, NVMe AHCI SSD Upgraded Kit for A1465 A1466 A1398 A1502 : Electronics
Amazon.com: 256GB SSD Hard Drive for Apple MacBook Pro Air 11" 13" 15" A1502 A1398 A1465 A1466 (Late 2013-2015) / OSX Sierra/Mac Pro A1481 : Electronics
OWC Tear Down of New 12” MacBook Reveals SSD Not User Upgradeable
MacBook Pro (Retina, 13-inch, Early 2015) Upgrade to NVMe M.2 SSD - YouTube
Surprise! iFixit says you can't do much to fix or upgrade the 2015 MacBook | Ars Technica