Never seen that. Perhaps because they're not mainstream there isn't a big enough market for them. And with the quality of mtb forks we see, probably not worth the risk anyway. Suntour GVX might be your best bet.
I have seen suspension stems and seatposts however. Knockoffs of the redshift stem and suntour seatpost can be found on Aliexpress. Those might be better options for adding suspension to a gravel build.
