Are there post-doc grants available for non-US citizens in the US?

There is relatively little funding of the sort you are looking for--funding that non-U.S. residents can apply for directly to fund post-doctoral work inside the U.S. However, that is not really that important, since that is not how most post-doc positions are funded anyway.

Most post-docs do not apply for their own funding. The funding comes from the institution that hires the post-doc (Very often--but not always--the money ultimately comes from a research grant from an outside agency, but the post-doc would not be involved in writing or administering the grant.) If an institution has funds for a post-doc, they will advertise a job opening and hire somebody. Generally, the search is global; they would be willing to hire somebody from anywhere in the world, provided they are a qualified. (There are some technical caveats about how this works, but they are of little import in practice; and--again--they are things that the hiring institution mostly needs to worry about, not the person they choose to hire.)

I think in some fields, grants like Human Frontier Science Program and Life Sciences Research Foundation are helpful especially biology. However, competition is very high in these and profile has to be in the top 5-10 of the applicants.