I see what you are asking going one of three ways:
First, 3D printing. Unless what you are seeking is already a digital 3D CAD design, you or a designer will have to produce one. A professional designer will charge for his/her services. A complex desogn could run into large bucks. Remember that your time also has value. Then there is machine time and material.
Second, a resin model generally good for 30 to 50 copies maximum. You or someone will have to produce a scratchbuilt physical model which can be molded and resin copies made. Again a professional will charge for their time and material. While you are scratch building the mold master, why don't you just scratchbuild your model and skip the resin casting step
Third, injection molding. Here you are talking big bucks Production costs are amortized over production runs of thoussands to tens of thousands. You can go two ways here. First you need a digital 3D CAD design that can be loaded into a digital mill (see number 1 above) At a time and material rate. The mill will cut the injection molds the you take them to an injection molding company to produce the plastic at a time and material rate. Second is the old school way, produce a physical model ( see number 2 above) and hire a die cutter to pantograph the part(s) into an injection molding. Time and material. Then like above you take your molding dies to an injection molding house to produce the model at a time and material basis
It will get expensive regardless of your choice. Your time has value. If it is a fantasy project of yours it may help defray the costs. I know I've spent months collection references for, and producing scratchbuilt model masters for a resin ship model company. Never made near my money back. I did it because I felt the subjects were unique and their was no model on the market.