OP:
There really aren't any easy or cheap options to upgrade the SSD due to the proprietary design that Apple uses.
As mentioned above, you can try to hunt down a used Apple drive on ebay. They won't be cheap.
Does OtherWorldComputing offer one that will fit the 2015 models? (not sure)
I've heard that the OWC drives run hotter than the factory drives, possibly other limitations, as well.
Another way to increase storage:
Get one of the customized micro-SD card adapters that fit into the SD slot.
Be aware that they will be FAR SLOWER than a real SSD, and will appear as a "second drive icon" on the desktop.
BUT -- properly managed, they can give you considerable extra storage space that fits "flush" into the body of the MacBook.
These could work well for large libraries of pictures or music or movies.