What do you mean you don't have it?
Everyone who has TF2 installed has all of the base game models installed, also make sure you are running the TF2 version of hammer editor, not the HL2 one.
I assume you are using a prop_static or prop_dynamic and you are trying to find the model, so click on "World model" > "Browse" > Type in the filter "Resupply" and the props_gameplay\resupply_locker.mdl model will show up.