The baseball products in the Renderosity Marketplace are add-ons for the CoreArts M3 baseball uniform.  You can buy that at PoserPros or Renderotica. 

CoreArts also has some freebie textures for the uniforms in Rosity Free Stuff.  (Yankees, Red Sox, Angels, etc.)

The CoreArts uniform is well-made and worth the money.  It's a very complete product.  Included are left and right smart propped balls, gloves and bats, and catcher's gear, fielder's and catcher's hats, and left and right batting helmets.  The uniforms have MAT files that allow you make the jerseys long or short sleeved, the pants high or long, the undershirt collar low, high, or not there at all.  There are multiple textures for the uniforms, bats, and gloves.

I've used Wardrobe Wizard to fit the uniform to Hiro, David, Furrette, and Koji.  It worked very well, except the shoes.  (Shoes are always kind of iffy with WW.)

There's not much else out there in the way of baseball stuff.  Sports is just not that popular for Poser users.  DAZ has a set of sports balls that includes a baseball.  EdW used to have a nice set of free baseball gear for M2, but it is no longer available.  Royloo has softball uniforms for several figures at PoserPros.

And there's this place, which has inexpensive BVH baseball motions:

http://www.3dstep.com/