I don't think u can. They are very rare. Just wait for the fortune-teller to show up. You can obtain those seeds from her and on one occasion I got it from the theater. (watching a game video.

Pick up all the trash, and fruit everyday. Start planting flowers and some trees to beautify your town. When u find minerals, plants, or bugs take them to the museum. Do this plus the favors of the towns folk and you will indeed start getting new people into your town. Up to 10 on Android. Not sure for ios.