Most of what I’ve seen has been home-made stuff. Both spheres and hemispheres from craft store foam for moons and planets. Also some asteroids and odd shaped moons from irregular shaped chunks of foam. Some neat, but simple texturing with spackle or other putty and paint. Sorry I can’t be more helpful.
You might consider asking this question over at Star-Ranger’s forum. Those guys are the best source I’ve found for starship gaming related questions and are always very friendly and helpful. Might even be able to do a search over there and find a previous thread or manufacturer without having to ask.