Otaru has little choice for ski and snowboard shops, so you are much better off making a quick trip to Sapporo. There are outdoor shops in the Wing Bay complex where you can pick up apparel and maybe some gear. In terms of renting equipment, the Otaru Kitashiribeshi Ski & Adventure center in Wing Bay can sort you out. They have skis, snowboards and apparel for hire and they will also deliver the gear to your hotel in Otaru. You can also use rental services on mountain at the ski resorts.