Where to buy SSDs for Mac Pro 2019

For a model that was released in 2019, if Apple no longer offers those modules you have probably missed the window on upgrading the boot drive. However, you have LOTS of PCIe slots, and OWC will sell you PCI slot card in a number of different configurations:

https://eshop.macsales.com/shop/ssd/owc/mac-pro/2019

Internal Drives and SSDs at Macsales.com

They know their Macs and are easy to work with.

*I* don't know the specifics of booting that particular Mac if it has no primary drive.

I do know it has a T2 security chip and this often makes booting from an external drive or anything but the internal drive more difficult.

Without any sort of primary drive in the machine now, you'll need to have a way to format your new drive and install macOS. You may be able to create a bootable macOS installer on a USB flash drive using another computer. Then again, booting from that might be problematic.

Call the folks at macsales and discuss your situation with them.

And I'm sure other users will add to the guidance here for you.