Fuji excursion train

Hello there,

I've bought the JR passes recently. We will arrive Tokyo May 28 (Sun) around 6pm. We plan to take Fuji Excursion train to Kawaguchiko next day May 29. I am afraid by the time we go to the JR counter, it will be all sold out for next day tickets as we arrived late. I've booked the hotel in Kawaguchiko already.
I've searched on line that I should reserve seats first. I cannot find any website to buy the Fuji Excursion train tickets. I don't know how to reserve seats in US. If possible, I would like to use my JR pass for part of the trip to Kawaguchiko.

Please let me know which will be the best way for my situation.

Thank you.