What keywords to use when searching for a Maya plugin developer? I'm not very familiar with VFX production but I've been asked to do some research on companies that develop Maya plugins.

However, I'm having trouble locating such companies.

First, I tried Autodesk's website, hoping they would have an official developer directory, but they do not.  Edit: I did find the official Autodesk site, but the rest of my question still stands.

Then, I tried Googling terms such as hire "maya plugin" developer and "maya plugin" development companies.  However, I am getting very few hits.

How can I search for companies that do Maya development?  I do not want a list of Maya development companies (that would obviously be off topic); I just don't know enough about the terminology to perform a search, so I'd like to know which keywords have potential.
Your best bet is probably to look for developers of commercial Maya plugins and see if they are willing to do contract work.  Most studios have developers on staff to handle any project specific coding work needed.  It's a pretty specialized skill set and I doubt there is much of a market for doing it as consultants.  

That said, my (limited) experience playing with APIs in 3d graphics software would tend to indicate that any developer with experience working with 3d systems should be able to pick it up pretty quickly.  The concepts, when it comes to the programing, aren't that different from DirectX to OpenGL to SoftImage or Maya.  It's still shaders, geometry, physics, etc...