First make sure this works:
Go to your Sales_Data layout. Add the field for regional manager from Store_Info to your layout. If your relationship is set up correctly, you should see the regional manager field show the same data for every Sales_Data record for every store that has that particular regional manager.
If so, then you can enter find mode, specify some criteria in the Regional Manager field and perform a find to get all Sales Data linked to the specified Regional Manager.
The next step might be to set up a field with global storage and formatted with a drop down list of Regional Managers. A script can use the data from that Global Field to perform the same find that you did manually, but using the data from the global field instead. The script will not need the regional manager field to be present on the layout, but you may want to keep it in order to confirm that the script worked as expected.
For examples of scripted finds that use criteria from a global field see: Scripted Find Examples
It works perfectly, thank you! I knew I was overcomplicating things!