Could somebody tell me please a good place to farm for Breath Ring in Arboria?
I just found only one in Crystal Palace.

No.

In all seriousness, all areas in Arboria, with the exception of the Sacred Grove, have the exact same drop list. (In the Sacred Grove, you get the Gelidia drop list instead, and I don't remember if the Breath Ring is on that list.) I believe UnterBrae level 4 has the same drop list as well.

So, just go wherever in Arboria fights are the fastest.

Or, if you're playing a classic version (not the Remaster), there's tricks you can use to duplicate the item. (Note: If you're playing the DOS or Amiga version, I seriously recommend stopping and playing a different version instead. The version included with the ARPG is the Apple 2 version, which is a good version, and the Remaster is also a good version.)

Then again, in my experience, depending on the game and version, it seems that the Breathring either:
* Has a chance to cancel out the breath entirely for the whole party (in which case you don't need more than one)
* Doesn't work at all (in which case even one is more than you need)

The Breath Ring makes immune to Breath Attacks, so everyone needs one and this works fine.
The Nospen Ring protect from Spinners, you need only one.

I play the Remaster and in Arboria it looks like, the Brath Ring is a hard to find. After many hours grind with hundreds of fights, i´ve got only 3 Breath Rings. Now I travel to Gelidia, maybe i find there some of them.

In at least one version I've played, the Breath Ring makes everyone immune to breath attacks; the breath attacks simply don't work, at all.

In at least one other version I've played, the Breath Ring did not appear to do anything.

(If you were playing one of the classic versions, you could actually duplicate the Breath Rings you did find.)