What course would I recommend for somebody that does not have any experience with improvisation?
It depends on where you are at right now. Improvisation of short pieces of various length requires to first know chords, then 4 part harmony, then add a little of modulation in periods (8 measures), and expand them into short versets (16-64 measures).
Here is the possible order of study for interlude improvisation:
Basic Chord Workshop, Harmony for Organists Level 1, Modulation Workshop, and Organ Verset Improvisation Master Course.
If you know harmony, you can jump right into versets. They are free pieces 1-2 minutes long, not based on hymns, and perfect for interludes, preludes, and postludes. But you have to know chords, rules of harmony, and modulations first.
[Thanks to James who is tired of playing from the scores during church services]