For metal reflectors, look into how to do brush plating. This kit is one that should do the job.
For plastic reflectors the lowest cost way is use Spaz Stix Ultimate Mirror Chrome spray paint. It will make a mirror reflective finish on front surface, and on the back surface of transparent materials like lexan, glass, polystyrene, hard urethane resins, and anything else it will stick to.
But for either, to get a reflective finish the surface must be literally smooth as glass.