I would agree with Bern here. Selene's Memory will solve your mana problem, but your potential as a DPS is wasted with that. I would rather insert a penetration artifact in that slot.
There is, however, another alternative to using that artifact or spamming SP potions. The trick is to cast Lv. 12 Divine Bless on the Tactical Assistance stance. With Lv. 12 Divine Bless, your mana does not go down at all when skilling. But it depends on your ET (Anis, for example, do not have Tactical Assistance). You may also consider a Diablo pet which gives Lv. 12 Divine bless on top of AR/DR +1 and other stuff if you do not mind spending T-coins. (But of course, attack would be weaker than if you used a Noble Popo or Divino.)
Where do i get Moon Piece/soft cotton yarn/pure Essence??
You can get Moon Piece and other Kielce artifacts from Kielce favor dailies. Every 100 favor points, you get a random artifact item from NPC Cain. You may exchange unwanted artifacts for one artifact using artifact exchange machine in Kielce Shopping Street.
Soft cotton yarns may be farmed in Skeleton Dungeons 2F or Viron forests.
Pure essences can be farmed from Shelter of Resistance (you can get artifacts from there too sometimes) or from the Kielce raids like Kielce Back Alley.