I harvested a PCIe Gen3 x2 NVMe 1.3 SSD from my older laptop, the problem is that its length is short. I've held it in place with some tape, it should hold.
Series Mp300 Ssd 120gb 240gb Nvme Pcie Gen3 X2 M.2 480gb Hd - AliExpress
SSD PX400 NVMe PCIe Gen 3 x2 - GOODRAM
64GB/128GB/256GB/512GB 3D nand PCIe Gen3 x 2 NVMe PCIe SSD NE 2242 - KingSpec
Lexar NVMe SSD PCIe Gen3x2 LNM520 M.2 2242 TLC HDD 128GB 256GB 512GB Internal Solid State Drive for Laptop|Internal Solid State Drives| - AliExpress