When I worked at startups and smaller companies we have interacted with:
Global Unichip Corporation (I believe TSMC owns part of the company)
https://www.guc-asic.com/en-global
Open Silicon (they literally merged with SiFive to become OpenFive and made this chip in the article)
https://en.wikipedia.org/wiki/Open-Silicon
Uniquify
https://www.uniquify.com/
Socionext
https://www.socionext.com/en/
There are others but these are the 4 that I have dealt with.