If you don’t want to drive from the airport you can catch a bus to Hirafu and pick up your rental car there. (Make sure you have actually arranged to do this beforehand) This is a great option if you want to avoid the drive over the mountains after a long flight or the possibility of a storm. Busses to Niseko are quick, comfortable and efficient from either Sapporo or Chitose Airport. They run very frequently during the peak tourist season, winter. There is a Tourist Bus counter on the arrivals level in the Domestic Terminal of the airport, but in peak season it pays to have pre-booked your bus ticket to coincide with flight arrival times.
There are four bus companies running services to Niseko in the ski season, see the Powderlife’s Access Page page for all of the options and links. Make sure there is a bus departure suitable for your flight arrival time, and if not (or you don’t want to step off a plane and onto a 3 hour bus ride) staying at the Airport Hotel overnight is a good option (they have a GREAT onsen).
To book buses online, only WhiteLiner and Hokkaido Resort Liner have English websites.
Out of the snow season there are less options for buses with sometimes only 1 or 2 buses per day.
Once you have caught the bus to Hirafu you can also catch a taxi to Tanuki Lodge if you are meeting friends there.