Come fall look around for oversized Christmas ornaments. I've got a 6" ball I scavenged from when I worked retail I'm planning on doing some light putty work to clean up the surface of and paint as a gas giant. I bet you could find one up to a foot if you looked hard enough. Also those stupid looking lawn globes - they go up to 18" or so, can you get plastic or acrylic ones? Might be on the pricy side, but they're an option.