I live in Sapporo and take the 7:40 to NRT pretty regularly. Never had trouble taking it from the trains using either the 6:00am or the 6:15. Get to the platform early if you want a seat.

There are also some buses that go from Susukino (http://bus.ekitan.com/kuko/Kp20?t=0&k=15&w=1&b=829) but I don't think you'll find info on that in English. It's marginally cheaper, but it takes longer.