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I know it's an old topic. In case someone stumbles upon it, small parts like springs, screws, or even optical elements can be easily bought online. The two biggest and probably most useful suppliers are:
McMaster-Carr MechE components
Thorlabs, Inc. - Your Source for Fiber Optics, Laser Diodes, Optical Instrumentation and Polarization Measurement & Control optics, precision parts

With calipers one can guess which screw size it is.