I advise and mentor Startups in San Francisco Bay Area, and some of them, like you, are in the hardware sector, including Robotics and Computer peripherals.

With prototypes, I would say there are two kinds - representative (a miniature model) and functional (actually performs the task).

If all you want is the representative kind, there are 3D printer services - I googled for Mexico City and found some fairly easily. You would have to see if they are in your budget.
http://www.makexyz.com/printer/rockit-print

It is difficult to talk about functional prototypes without more knowledge of the product. Let me know if you'd like to get on a call.

This 3D printing option would give you a starter kit to take to investors - there are some great investors in the Bay Area that could incubate and grow Non-US startups. Would traveling to the US be an option?

More than happy to discuss further, this is a very admirable field you have chosen. Good luck!