You can make flat reflectors with glass panels or acrylic panels and apply window mirror film. Ordinary window glass might be less expensive than acrylic and many places will cut to size, but unusual shapes might not be an option. The mirror film is relatively easy to apply and the packages have instructions included.
For the hologram pyramid you referenced, one could 3d print the bracket at the base. If your design is different, yet similar, that's an option for relatively easy construction.
100-200 pieces is really too small of a quantity to justify expensive injection molding. If the part is not too complex, one could create the model with 3d printing, then build a silicone mold around it, followed by pouring polyester or epoxy casting resin to make larger quantities. A better idea of the shape would be useful, but the information I've provided may head you in a practical direction.