Many directories don't list such a category. Model makers are often the people who make architectural models and aren't appropriate for your needs so question them closely about what they do. And try to present professional drawings the kind that machinists are used to working with. It's advisable to work with a draftsman to create engineering drawings that have dimensions and tolerances. If you're unsure about how to determine shape, size, tolerance or material, consider buying an hour of an industrial designer's time for advice.
Most industrial designers work alone or with one or two others, and are inventor-friendly.
But be sure to ask what kinds of products the designer works on, especially if your invention will be manufactured in plastic when in full production. More Don't spend your life's savings to produce your prototype. Jack Lander is a protoyper for inventors. Prior to starting his own business, he worked for several years as a corporate manufacturing engineer and later, as a mechanical design engineer, acquiring 13 product patents. All answers are intended to be general in nature, without regard to specific geographical areas or circumstances, and should only be relied upon after consulting an appropriate expert, such as an attorney or accountant.