Yakushima Car Rental

We plan on taking the jetfoil to Yakushima where we want to rent a car. The only companies we can find with English websites are at the airport. We don't speak any Japanese. Are there any companies located at the port with English websites? Can we just show up at the port and get a car on the spot? We are travelling with our 4 month old baby, so we will also need a car seat for her.

I received a lot of information from Yakumonkey before my trip many years ago, you may try to contact them:
http://www.yakumonkey.com/p/contact-yakumonkey.html