they are not borrowed from anywhere. its quite normal for 'out of key' chords to be used in songs. E major and D major or D7 are quite common in songs in the key of C less so for A major but quite acceptable remembering songs I play. the six chords C,F and G major and D,A and E minor aren't the only ones you can use in the key of C. Bb often crops up too......

most guitarists are used to these chords forming part of the backing - do you play any instrument?