Can anyone tell me for what reason does the needle setting on this motor's carb just moves. I have tried two carbs already and now I on the look out for a replacement carb. Is there a known problem with the Ofna hyper .21 8 port race engines?

It's Must be Worn O ring on the needle,change with New O ring or You can try to soaking the O ring with gasoline.
Hope this help

Where do you get replacement O rings for the Carb needles? And will the gas expand out the worn ones?

Remove the carb and seal it where it attaches to the engine block. Do the same for the back cover. These are known air leak spots.

As for the O-rings, try Ace Hobbies, I just bought some carb O-rings from there today and they had two more bags on the shelf.

Yes, it will expand a little..