You want the large bark mulch. Big pieces (4x2x2 average), stuff actually looks like rock, especially painted up.
Home Depot, and especially Nursery/Landscaping stores.
Can also find it free. My house in Fish Hawk had a stupid HOA rule that you could ONLY use said mulch, in brown color (Which I defied)
Also, anywhere they are cutting big pine trees, or firewood places.
Probably, craft stores like Hobby Lobby have even bigger pieces?