Pre-edit: I just realized I never actually answered your question. I honestly don't know how most people go about finding someone to design and prototype, but judging by some of the responses here I think we'll get some good advice. Feel free to contact me directly if you want me to look into the draft.

What has been super helpful to me iwhen people come to me with requests is to send me the most detailed diagrams or images of any currently available offerings or a combination of them so I have a good starting point.

As a specific example, someone asked me to design a handheld door lock retainer to practice lockpicking. The overall shape and ergonomic design were up to me, but this image was provided which included very specific measurements for the lock:

If you can source an stl file or similar for what you want printed. For really small batches, there are people on Etsy who do commissions.

Check your local library system. One of mine has a 3d printer for use.

Getting a model printed is pretty straightforward. There are many online services where you can send a 3D model file and they mail you a print of it. The bigger challenge is the design. Paying a professional to design something for you is going to be very expensive. However, many 3D printing enthusiasts design their own models as a hobby and make them available for free. I would suggest looking on sites like printables and thingiverse for something that suits your needs. If you can find it there, then you can just send the file to a printing service and have it made. Other options would be spend time to learn modeling/design yourself, or find a kind person to do you a favor and design something custom for much less money than a professional would charge.

There are ppl on eBay who will print your stl file for you. I've had luck asking sellers who does their stuff.
I sew a bit so ik a small bit about bobbins and idk if a 3d printed one would work well or hold up.