Amazon.com: CMS 128GB mSATA 6GB/s Internal SSD Compatible with Lenovo ThinkPad T420 Series - C29 : Electronics
BEWARE: Lenovo Thinkpad L15 Gen3 SSD is practically un-upgradeable. Laptop comes with 256GB and bigger SSD is too thick to fit. There also goes a cooling plate on top. : r/thinkpad