By getting the silver and copper pigs in treasure hunts. Copper and silver pigs appear also in normal shop for coupons during some events, just take a look at it every week.

That said once we get kagw ti normal and hard mode (very likely within next month) those tis will drop also ninja tool essences (the same as ut) as secondary rewards (so in the place of natures, eventually).

Is way easier now to get 32k power with the nerf of the chance than before. The only stats that matter for the ninja tools right now are elemental resistances, but since we won't get back the +4s until kage ti normal and hard mode are unlocked then ninja tools do not matter in any way more than other sources of basic stats (overall because now the links got a nerf and went back to the 2.0 10% from the 4.0 30%). That said i understand the problem but you still get 3/4 tools in average per week it is just harder to reforge but, as i said, there is no reason to reforge until we get kage ti hard mode, so do not worry, you are not losing anything significative.