replacing the siphonic part in the toilet?

I need to replace the siphon in the toilet.The siphon is a 3 pt unit and I cant seem to find the main part in store.I don't want to take the system off the pan as I'm no plumber.I have taken the siphon out of the toilet to see iff I can replace the main part that attahes to the handle and I can get it back in again and sometimes it works sometimes it doesn't.