I took a class based on Getting More and it was interesting and useful. This was offered by the company so I can’t comment on general classes. There’s evidently an online course based on the material, no idea if it’s more useful than just reading the book.
I like GM more than e.g. the Carnegie book because it’s just usefully framed as “understand what the other person wants and try to get it for them” which is like 90% of being a good negotiator, assuming you know what you want as well.