how to purchase simple custom devices with low quantities?
I think the other answers are missing the mark. If I understand the question correctly, you're not interested in doing the engineering and prototyping/manufacturing yourself; you're looking for someone to do the design and prototyping for you and then help you transfer the design to a contract manufacturer if/when it goes into production.

That's what an engineering consultant is for (and that's what I do for a living). This person (or firm) will take your functional specification, which you've outlined very nicely, help you refine it, then find the right electrical and mechanical components to implement it, build and test the prototypes, and in the end, provide you with all of the documentation (schematics, bills of materials, assembly instructions, user manual, etc.) required to produce it in quantity.

There are some contract manufacturers who provide this sort of design-build service in-house, which means that they'll do both the engineering and the volume production for you, and you don't need to deal with two separate entities. The types of assemblies you're talking about would actually be a good fit for this model, since it doesn't sound like you need a lot of very specialized engineering knowledge for your adapters.

So, look for "contract manufacturers" in your area, and see which ones have in-house design services. There are frequent trade shows for these services all across the country, too, and this is a good way to see what's out there. "Design-2-Part" is one series of them I'm familiar with.