The easiest way to source a 9th is a whole step above a Root note (often the octave above the starting Root note). Adding it to a major chord gives the add9 chord. Other ninth chords will include a seventh and, therefore, (theoretically) will contain five notes (R-3-5-7-9). The type of 3rd and 7th will determine what type of ninth chord you’re playing.
Since the chord contains five notes, it’s not always possible to play all of them. The first note that often gets removed is the 5 as is does not add any harmonic information. You may find too that the Root is often omitted as you get into chords with more extensions.
Hope this helped you 🙏🏻
