You have 5 options in your situation:
1. Buying a
Apple SSD -
You can buy a second hand/OEM drive at your own risk from an eBay or Amazon marketplace seller. The 2015 Apple SSD speeds are the fastest on the market for this model. Careful looking at third party sellers because you could be paying a huge amount for a slower SSD from a 2013/2014 macbook pro
or
You can buy one new from an Apple Authorized Provider, but the costs are so high. I contacted a provider and was told with taxes included, a 1TB drive from Apple would cost around $1200US (I'm not kidding)
Apple Store will not help you with upgrades. It is either paying a fortune for an upgraded Apple SSD through an Apple provider or buying used online
2. Buying a
OWC Aura SSD
OWC has a 480GB and 960GB flash storage option ($380-640) BUT this SSD is much slower than the SSD in your current macbook pro
https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015
3. Buying a
MCE Technologies
MCE offers a 512GB and 960GB flash storage option ($449-649), and they claim to offer 1440mb/s read and 1100mb/s write, a little closer to the speed of the Apple SSD
http://store.mcetech.com/mm/merchant.mvc?Screen=PROD&Product_Code=1TBPCIESSD-L13M14
4. Buying a
used or Apple refurbished 2015 MBP
If you are set on staying with the 2015 MBP generation, there are still refurbished models popping on the Apple refurbished store and you could buy an upgraded mac and sell your old mac. 2015 MBPs still hold a pretty solid resale value given the controversy over the 2016 macbook pro.
Refurb me is a great site to subscribe for alerts on the Apple Refurbished Store
or
Buy used on ebay or Amazon
5. Buying a
MicroSD card & adapter
This is the most affordable way to add more storage to your computer
You can easily add up to an additional 256gb storage to your computer by buying a micro sd card and there are adapters that are designed for 2015 MBP to run flush with the computer so it doesn't look like anything is sticking out.
The down side is that micro SD cards are much slower, but this would be a good option for storing iTunes media and freeing up space on your SSD for other uses where SSD speed could be better utilized
https://www.amazon.com/SanDisk-Ultra-200GB-Micro-SDSDQUAN-200G-G4A/dp/B00V62XBQQ/
https://www.amazon.com/8MOBILITY-Aluminum-MicroSD-Storage-Adapter/dp/B00TTFOH5Q/
Hope that helps