Is there a formula/theory for writing country licks/riffs, and where can I find it?
It's the same formula/theory to write any music: listen to it. Listen to as much as you can and get it in your head. I have been trying to write and improvise solos on guitar for country and it is really difficult coming from a rock background. Country guitarist use reverse bends, major pentatonics and play with the chord changes targeting notes of those chords in the songs. So I had to really listen hard and practice starting with Johnny Cash guitar licks.