When you need to perform a find and the field you wish to use is in a sub summary part, there are several options:
1) use a script to perform a find. Set field will work to put criteria in the fields regardless of whether or not the field is on the layout.
2) Put a second copy of the field in a different layout part. Use "hide when printing" to conceal the field when you print the layout.
3) Perform your find on a different layout based on the same table occurrence. Layout's base on the same table occurrence (They have the same exact name in "show records from" in layout setup) share the same current record and found set so you can find and sort your records on a layout where the fields are accessible, then change layouts to the report layout.