Mechanical or steampunk is a theme or subject matter. Good artists rarely limit themselves to one or two special themes, certainly because that would get pretty damn boring year after year. My recommendation is that you learn something about different styles (photorealism, neo-traditional, illustrative, new skool, etc) and then pass your overall idea and subject matter by an artist that excels in the style you want.
Aaron Bell specializes in the Japanese style, so he likely wouldn't be such a good target or fit, but there will be many, many artists of each style up in the PNW.
Something else to consider - any great artist probably isn't going to have much if any interest in exactly reproducing your friend's artwork. The exception to that "rule" might be artists who specialize in photorealism, but even then you're friend is going to have to be one helluva artist himself. Your inspiration piece should likely be considered only as such - a reference (amongst others) for what the tattoo artist is going to create just for you.
