Sadly, other than finding a used Apple SSD, OWC is it!
We tried looking for an Apple SSD for our first two systems of the three SSD's we tried to buy off the web only one worked but it was the wrong size from what we ordered!
The larger Apple SSD's are very difficult to locate as they are a custom part. We now order the 1 TB version of the system and don't mess with the internal drive any more.