You can probably replace it with any 1TB NVMe M.2 SSD you find at any retailer. You can find plenty of those on Amazon.
Tip: Look for the performance models, such as Samsung PRO or similar.
@bthorne As long as it's the same connection (not sure, but maybe you should look for NVMe M.2 SSD) it should work without problems. You should run a quick search on the internet to check if anyone has found some sort of "blacklisted" parts (some vendors such as Lenovo used to place some code in their firmware which would prevent the computer from booting with a "unauthorized" wireless card for example.).
However I think this is not the case with the Surface Pro 4 or the world would be at rage right now. Just look for any size NVMe M.2 SSD you think it would be fit (if you can afford, look for performance models such as Samsung PRO, there's a huge difference from the cheaper ones.). Hope it helps!