I bought them at a local craft store. I've been searching online for a good source but no real luck there.
I would be willing to paint up planets by hand as commission work.
Thanks for the comments.
Prep time: (10-20min) After sanding the bottom side to make smooth and washing with soap.
Prime: (5-10min) I used the Vallejo gray primer
Paint (60-120min) hand painted.
Varnish (?) I still have to do this.
All said and done I figure it takes under 3 hours of work. Thinking about what to paint takes longer.
I also plan to paint up a flat circle of the planet to place under it, so while playing you could remove the planet during play when needed.