mac spotlight does not find all files, and there is no way to tell it to search
a directory literally (i.e. instead of using its index). this will get you all
of the files you want to examine, in one place:

cd /path/to/folder
find (whatever criteria you're looking for) > file_list
gnutar -vcf ~/Desktop/files_of_interest.tar --files-from=file_list

Then, you can extract the tar file and flatten it.

alternatively, you can create symlinks to every file in file_list in a flat
directory somewhere else, but this has the disadvantage of obscuring file
modified times.