You will need the MESH_ju-beos_slimbb-shorts-bottom060312 mesh from
here for these to show up in your game.
Here are 16 pairs of shorts, as bottoms only, for the slimmer bodybuilder.
They are organised into 2 sets with 8 pairs of shorts in each set.
Set 1.
Set 2.
Credits:
Mesh - Marvine and Beosboxboy