You can download and use the IMIS for free.
If you use it, please quote the appropriate reference: Floridou, G. A., Williamson, V.J., Stewart, L., Müllensiefen, D. (2015). The Involuntary Musical Imagery Scale (IMIS). Psychomusicology: Music, Mind and Brain, 25(1),
You can also download the scoring template, where you can import your data and get the scores for the four factors quickly and easy.
The IMIS can be used along with 3 additional items that are related to INMI but not part of the scale: frequency (1 item) and duration (episode and section; 2 items). It is advisable to use at least the first item related to INMI frequency, as a screening question to identify individuals who never experience INMI).