I am in need of a Windows DLL library that will, given an MPG input
stream, decode a single frame to an RGB buffer. It does not need to do
anything else (except I guess get the size of the frame so I can
allocate the buffer). Where can I find this library, and if this
library is not available, who can produce it for me?

I believe Lead Technologies has what you're looking for. They sell a
development kit called Multimedia Imaging Pro:

http://www.leadtools.com/SDK/Multimedia/Multimedia-Imaging.htm

There are more details here:

http://www.leadtools.com/SDK/Multimedia/Multimedia-Products-n.htm

Scroll down and note the following:
"Input support for disk, Internet memory, media source or custom (feed
every sample or write your own source filter)."

Sounds like that's just what you want.

Trevor