You can use the Get Unallocated Elastic IPs to get your Unallocated Elastic IPs by region.
However, if you need to get all the Unallocated Elastic IPs for all the regions then you need to first get all the regions and for each region fetch the unallocated Elastic IPs. 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.