I bought the 8x8 matrix from Monoprice - they have a category for audio/video splitters. They brand it as the “Blackbird” line, but note that they rotate and update the OEM/manufacturer fairly regularly. They’re clearly rebranded OEM units with a very cheap web interface and IR remote.

The 16x16 I bought from a small online company called “HDTV Supply” - model number HDCVT4K1616. They were a pleasure to work with.

(Mods please let me know if there are any issues sharing these vendors.)