If the lamp is a standard size, then the easiest place to find the mounting parts may be a truck stop or truck/trailer repair shop.
Most all of these round lamps are designed to fit in a rubber grommet that just pops into a flat panel (with a liberal amount of soapy water). There are some round lamps that can be screwed on.
The stop/tail
lamps
The marker/clearance
lamps
Jason