Has anyone ever created a

FOLDER

selector in NetSuite?

I don't think there is a native way of doing that. Would probably have to go the custom route. Perhaps getting all folders with a search, build the hierarchy and use some package for generating a nice selector and then store the internal Id of the selected folder in an integer field on your record. I haven't done that, I usually resort to just require the admin to enter the full path or the internal Id of the folder.