I'm aware of only two. Airfix makes a model in 1/144 which I have not built, but I would imagine it's pretty straightforward, if small. Others would know more. there have been lots of decals and parts made at one time or another.
Mach 2 makes a kit in 1/72, which I do have. It is a very crude and difficult kit, even by Mach 2 standards. There is very little surface detail, which really shows on a model this big, and there would be any number of enhancement projects to get all of the things like gear, engines, wing fences, doors etc. even halfway respectable. The clear bits are not well cast, and the decals are limited in choices, incomplete and as far as I can tell almost to nothing is out there in the After Market. When I do build it, I'll plan to clear the bench and really stay focused.
It's a classic though and United operated a lot of them in the 60's, so they were well represented at midsized airports.