You can use the Amazon EC2: Get Volumes plugin step to get all the unattached volumes. You can apply the status=available filter to get the unattached volumes.
You can use the Get Volumes step to get your unattached volumes by region.
However, if you need to get all the unattached volumes for all the regions then you need to first get all the regions and for each region fetch the unattached volumes. You can design this by using parent child workflows or loop steps.
In the images below I am representing the solution using parent-child workflows.
The image below shows the parent workflow to fetch all the regions.