afni.nimh.nih.gov

The datasets that end with "mpm" are "maximum probability maps", really the maps of the regions that give the maximum probability at each voxel, rather than a specific probability. You could use a ventricle mask from the N27 dataset in MNI space. Download the N27 surfaces and volumes in MNI space here:

[afni.nimh.nih.gov]

You could use the ventricle mask dataset, aparc+aseg_REN_vent.nii.gz, as a mask. Take a look to see if that's what you want.