There’s no way I know off to query for the IMDB ID directly
You need to do something like:
GET /library/sections/<KEY_OF_SECTION>/all?X-Plex-Container-Start=0&X-Plex-Container-Size=50
Above will return the first 50 entries
Get the key value from each of them one by one, and issue a GET on that, like
/library/metadata/51998
Then get the next 50 items
GET /library/sections/<KEY_OF_SECTION>/all?X-Plex-Container-Start=50&X-Plex-Container-Size=50
