If you want to have other models from Source in your CS:S map (generally not a good idea in the first place because of differing styles - TF2 is cartoony, Portal is well, Portal, etc. DoD:S seams feasible, alien swarm is futuristic, etc.
The only other one which works is Half-Life 2, but that content is already included with the standard CS:S config.
However, if you absolutely want some cartoony models mixed in with the hl2-styled CS:S map, be my guest.
Google GCFScape and download it, navigate to C:\Program Files\Steam\Steamapps, and open up any one of the gcf files with GCFScape. Drop whatever model files you want in your CS:S folder (default C:\Program Files\Steam\steamapps*steam username here*\Counter-Strike Source\cstrike)
NOTE: Folder structures must remain intact. You can’t have a model file in a different folder than the original - eg. /models/random/somemodelname.mdl can’t be in /models/dods_models/random/somemodelname.mdl
SECOND NOTE: When you copy a model, you need to copy the textures over as well, those are also folder-sensitive.
And why did you create a CS:S mod if you aren’t making a mod?