I guess I've not even checked general stores recently, but I'd assume at least somewhere sells them. Have you tried some of the new areas' merchants (they'll seem sort of obvious and out of place and often won't be on the world map)? I remember there being a glitch this past winter that affected some stars being recharged at vendors, so something very well might have come up recently.
Assuming none of the general merchants/potshops sell stars:
As soon as you can do Monster Park, do that. Stars drop like candy.
Check through your monster cards for things like Kumbi, Tops, or Icies. Last I remember, a stars have a pretty high drop rate.
If you're really desperate for free, good stars, hang out in Monster Park lobbies and loot ones people discard (this happens frequently).
Worst case scenario, Nexon is being terrible and doesn't recognize the problem (assuming there's a problem), and you might have to play a different class for a while. You could use this time to make a link mule or two to buff your future Assassin.
If you don't get any definite responses, post again, and I'll go on and search through various vendors for you; it might be difficult for a new player to find all of the possible vendors.