here are some information.
By Shibazakura Liner shuttle bus
A shuttle bus, called the Shibazakura Liner, connects Kawaguchiko Station and the Fuji Shibazakura Festival's venue (30 minutes, 1-2 departures per hour) during the festival period. A 2000 yen ticket includes the round trip and the admission fee.
By regular bus
There is one regular bus every 1-2 hours between Fujisan Station, Kawaguchiko Station, Lake Motosuko and Shin-Fuji Station. During the festival period, some of these buses make a stop outside the festival venue at the Fuji Shibazakura Iriguchi bus stop. The ride takes 50 minutes and costs 1230 yen from Kawaguchiko Station, or 75 minutes and 1710 yen from Shin-Fuji Station.
And you can buy the Shibazakura Liner in shibazakura Festival site. I suggest buy the ticket once you arrive there cause it would be very crowd or the ticket sold out and you may can not buy it.
Hope you have a nice trip in Japan.