Sounds like a perfect application for a small desktop 3D printer. I would recommend you invest one and the software required to create designs for it. You will learn A LOT about how these things are done. Plus you would be able to develop and refine the shape of your prototype at very low cost. I know of 3D printer kits you can get for less than $500. One source is reprapguru.com. Once you've finished massaging and adjusting your design you can approach professionals in the molding industry on how to proceed.
I will leave it to others on this forum with more experience in the world of injection molded plastics to answer other questions.