I believe the bipod (together with a black-painted beech stock, no sights, and a muzzle weight) came as part of the package offered for a few years as the Diana 48 Black Pro about (?) ten years ago. Maybe some bipods were sold separately?

I am not a fan of bipods on anything much smaller than a heavy sniper rifle or a Bren gun. If you want to shoot prone, learn to shoot prone. Just airing my prejudice.