Furano has a few ski & snowboard rental options. Depending on where you are staying, some rental companies maybe more convenient than others.
Harro Snowsports is an Australian rental company that has shops in Jindabyne and Furano. Their staff are fluent in English and have a good range of equipment. Their shop is a short walk down the road from the Kitanomine Gondola Building. http://harrosnowsports.com.au
Good Sports is a another good option for guests staying in Kitanomine or locations other than the New Furano Prince Hotel. They have a shop opposite the Kitanomine Gondola Building and offer a discount for people booking their rental in advance on the internet: https://www.goodsports.co.jp/white_eng/rental/
If you are staying in the New Furano Prince hotel, the easiest rental option is probably to rent directly from the Atomic rental shop in the hotel. They also have other shops dotted around the resort.