The closest thing to Prog I've got from the US is Ars Nova's self titled album from 1968, sounds like nothing else I've heard from that era out of North America
Ford Theatre are also worth a listen, I see these getting referred to as Prog but it's West Coast psych to my ears, regardless though their debut Trilogy for the Masses is essential listening.
These are all acts that recorded their best stuff around 1968 so maybe a wee bit early for what you're asking for in your thread starter, but there's something about Ars Nova and The Mandrake Memorial that's closer to UK prog than their US contemporaries to my ears