Sintech sell adapters via amazon US/UK etc for about $15 that allow you to take a regular NVME drive (which are VERY cheap just now) and adapt it to the apple SSD hardware interface. You can have up to 2TB of faster-than-ever storage. You'll also need torx screwdrivers to open the case.
If you do some googling you will see there are two versions, and depending on the size/shape of SSD you are buying it may be better to buy one rather than the other.
If you do more googling, people have tried various drives and report on performance/compatibility etc but generally compatibility is good except with some samsung drives.