I looked up the Hokkaido Orienteering Association website.
http://o-hokkaido.com/
They have one on Sept 21st right on the outskirts of Sapporo. It's accessible by bus or subway. The map is oddly named 'Olympic Tree'.
There's also one on Sept 20th (Park-O) in Takino Suzuran Park, a tiny bit further out. Accessible by Subway and then bus.
As it happens, there's also one tomorrow.
If you want more info, email me.