Recently i have been finding chigaiya magatama for my character, however i can't seems to find it. Anyone can tell me where can i find it?

The 3 Gankubi brothers in Oosumi Plains West give it to you in a quest regarding eyedrops. You need to clear part of their quest chain before the quest is presented to you (it's the 2nd or 3rd in the chain) and it may require you to clear the quest involving Putrid Gastric Juice (although I'm sure it doesn't require it cleared for the quest to appear, rather you just need to clear it before you can finish the quest), and it requires some extent of storyline completion.