You can either get an original SSD from eBay or buy an after market SSD from OWC https://eshop.macsales.com/shop/ssd/owc/macbook-pro-retina-display/2013-2014-2015
It shows that TRIM is not supported... Is there another SSD that supports TRIM? Do you know the specs of the ssd so I can search for one? Thanks!
SSD from any MacBook Pro Late 2013 or later, MacBook Air Mid 2013 or later, or Mac Pro Late 2013 would work.
Search on eBay "SSD MacBook Pro Retina". I am sure you can find some. They are available in 128GB, 256GB, 512GB, 1TB. Not sure about the specs, but as long as it fits, it would work.
Only OWC make an after market SSD that fits apples connector and form factor. So it's that or buy a used one pulled from another Mac of the same vintage to keep the ultrafast speeds.
At the moment there are no upgrade paths that withstand much scrutiny. It might be better to simply accept the computer is non-upgradeable and instead look ahead to its replacement.
The problem with any non-Apple upgrade for a 2015 unit is loss of performance. The Apple units are faster (1.5 GB/sec) than third parties (0.4 GB/sec). And Apple uses a proprietary interface that other companies have to re-engineer.
A bit of googling would answer that quite fast. Apple uses a proprietary PCIe form factor and connector. It's either a used part bought on eBay or a slower one from OWC, your pick.