If you'd like to search for an item that's related to a particular order but can't remember which order the item is related to, all you know is certain keywords you included in the order's custom fields. You can search the item report by that particular custom field.
If you need a refresher on what custom fields are and how to use them check out this article.
To do so navigate to the Order > Item Report and click on the Filters.
Scroll all the way down where your custom field search options are located. Select the appropriate custom field you would like to search. In the example below I would like to search by the few words I know are in the custom field, so I selected "contains" as my search parameter.
The information that is pulled from this search is a list of all the items related to the order that has the words "PO Box" in the Extra Billing Info custom field.