A quick search for baseball border stencils on StencilSearch.com netted a great selection! Any of these would look great all around a room or on an accent wall, such as over a headboard in a bedroom.
Stencil Planet has a fun baseball border that comes in a few different sizes and can also be custom sized.
The Stencilled Garden offers this fun baseball cap and ball repeat border design.
Designer Stencils has this great baseball themed border.