I have used Meyer et al's (1993) measure of commitment based on the Three Component Model. It works fine, but as with many measures of commitment, it includes items that relate to turnover intentions and organisational identity. I have also used Klein et al's Uni-dimensional Target-free (KUT) measure, which was published in 2014. It measures commitment solely in terms relating to dedication, but it does avoid conflating the construct with other variables. Because it is target free, it is a simple matter to just add in the phrase that will specify what the target is (e.g. an organisation, a profession, volunteering itself, etc).
Prior to searching a scale you need to justify taking volunteers as sample for organizational commitment measurement. Volunteers usually do not stay that long with an organization that there commitment can be made and measured.
There are NGOs depending solely on volunteers (for achievement of organizational objectives and running operations) with few employees and they do have long-term volunteers too although volunteer retention is much difficult comparatively....
As understand its better to frame a model for assessing commitment of volunteer. its better to prepare by self. If you require we can collaborate to make it happen. Do the ground work of content selection.
I agree with Mr. Clement Allen Meyer's scale of organisational commitment has been validated in different settings and countries and results are consistent.
Allen, N. J. and Meyer, J. P. (1990), The measurement and antecedents of affective, continuance and normative commitment to the organization. Journal of Occupational Psychology, 63: 1–18. doi: 10.1111/j.2044-8325.1990.tb00506.x