Unfortnately, you can't get anything in the game with Luna. The currency is mostly Time Elixirs or potions. 4* usually go by 1TE, or 2 or more if they are limited edition (LE). Potions are also currency, and they are 2Pot = 1TE. You can sell your maxed (lvl 80) 4* and get 4 or 5te for them.
5* are much more expensive, starting at around 8 - 10TE the most common ones, and going up to 50 - 100 the rarest ones.
6* usually sell 70+ TE each, with some going up to the thousands. It usually takes a month or more to get the first 5*.
It is also common to exchange 3 maxed 4* for a 5*.
It takes some time but it is doable for free.
If you have any TE you might want to look at my 4* shop and maybe I can sell you one of those if you find anything you like
You can open a trade in game with the ID that appears on the left sidebar of people in the forum.