You can find large multi sport outlets in the city which carry a large range of brands and items. There also a couple of smaller independent shops worth checking out for local snow brands and fashion.
Shugakuso is a ski focussed retailer with a large range of skis from local and international brands. They also carry touring, telemark and back country equipment.
For snowboard gear, check out Murasaki Sports located in the Aeon mall in West Asahikawa. They are Japanese chain dedicated to board sports.
Spray is a local Asahikawa snowboard / skate shop deeply embedded into the local culture. They have a large range of snowboards designed for Hokkaido powder from international brands like Salomon, Korua and local brands like Gentemstick, MOSS, Field Earth and Vector Glide. Fine outdoor fashion and accessories from domestic labels. The shop also has the largest indoor skate park in Hokkaido.
During the ski season, White Time has a pop up shop in the OMO 7 hotel with a selection of gear from Vector Glide and Blank outerwear. You can also arrange for demos and waxing.
Kona Surf is a snow / skate shop run by professional snowboarder Shuhei Sato.
Asahikawa Ski & Snowboard Rentals
Most Asahikawa resorts have rental stations. Kamui will keep your gear overnight for multi day rentals. There are also a couple of small rental shops in the city.
Asahikawa Ride is guest house for skiers in central Asahikawa. They also provide outdoor rental like skis and backcountry equipment. If you are looking to rent powder skis and backcountry equipment you can contact Mush Rooms in Higashikawa. The lodge has gear you can rent for off piste adventuring.