Bus from Mishima to Kawaguchiko

I am travelling from Shin Osaka to Mishima by shinkansen and then to Kawaguichiko.

I would like to take the Fujikyu bus Mishima to Kawaguchiko on Sat 28 Nov as I will be taking part in the fuji san marathon on the Nov 29 Nov. I notice that there only a few buses to Kawaguchiko on the saturday. Is this a very crowded time at Kawaguichiko ? Will the buses be fully booked?

Would I be able to get seats on the bus on the spot at the bus station or would I need to reserve in advance?

For buses, do I buy tickets on the bus or from the bus station?

Thank you for your help.

1)
Now NO full booking all.

2)
Only by Call if in out of Japan.
Fujikyu Call Center TEL: 81-555-73-8181 (Operation hours 7:30 to 20:00/JST)
Already booking possible.
http://www.fujikyu.co.jp/citybus/highwaybus/pdf/Mishima_Kawaguchiko_Li...

3)
Possible at ticket sales window of Mishima sta. if sold out yet.