So you have 2 options here (well 3 actually!)...
1. is to create your own polygon file from a Shapefile. There are various tools to do this (eg. Shapefile to Tableau TDE – Free Online Converter | Safe Software ), and I've written up another such way here (as you can turn the file that created the boundaries, as custom polygons, into a TDE you can have this on public....it's just a data source)
2. In the above link, there is also a link to an alternative method (which I've just checked and seems to work in T10), where Craig Bloodworth, has created these levels (there is a video on his site on how to install and enable it). This may have been what you've seen before. btw as he did this a few years ago, if any boundaries/names have changed they might not get picked up
3. With Tableau 10, we have custom-territories, so potentially you could build these from PostCodes. Although this video covers some other T10 features there is a great explanation on how to use custom-territories.
Hope that helps.