I was in the same seat as you a couple of months ago. I emailed basically every single shop and manufacturer I could find. This is what I came up with.
There are 4 high profile European/Almost European manufacturers whom openly produce SAPI cut plates. Rabintex (Israel), Mehler Vario Systems (German), NFM Group (Norwegian/Polish/whatever) and TenCate.
According to the morons over at BestProtection, Mehler Vario Systems does not sell SAPI cut plates to non-military/LE, because "it's just not possible". Even though they sell Level 4 plates of other shapes to basically anyone....sigh.
NFM Group also only sells to Military/LE.
I have not checked with Rabintex yet, and I don't plan to.
And now to TenCate. TenCate has no issues with selling SAPI cut plates to non-military/LE, however, they do not sell to individuals. And pretty much every supplier of TenCate plates ONLY sell plates to Non-Mil/LE personel. All but one, Varuste.net (Finland). I emailed back and forth for months, and eventually got them to sell Level 4 SAPI cut TenCate plates. But they did so at the outrageous price of ~1000€ PER PLATE.
They have since removed the SAPI cut plates from their website. But they do still sell SAPI cut soft armor. I just combine the SAPI Cut soft armor with regular 10x12, and it works fairly well.
If more people were to put pressure on the different manufacturers to produce SAPI cut plates, something might happen. But they do not seem to be the brightest of minds. Basically every single modern plate carrier is designed to carry SAPI Cut plates, yet the ******* morons who produce the plates refuse to simply change the cut of the plate, probably losing potentional sales as a result.
You can also check German Ebay from time to time, but any plates who show up there will to 95% be stolen.
Sorry for the long text, but I felt it to be necessary.