With the Find in Files window showing, change the Look In: combo box to anything except for open docs and current docs.  This will enable the Look at these file types: option to select what files to search for.

FindInFilesSearchInFiles

And for more advanced searching…

There’s a "…" button next to the Look In: combo box. 

Choose Search Folders Button

Pressing this will pop up the Choose Search Folders.  Here you can create a set of folders to search in.

Choose Search Folders Dialog

And now you can find your custom folder set as a Look in: option.

Custom Folder Set as search option

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