If you are storing the string as an external_id on the item use filterItems to get the item with that external_id. If you are not storing your string as the external_id start doing so :)
The exception being: If you are manually creating these items you cannot set the external_id. If your string is numeric save it to a Number field and use filterItems to find it again.
If it really is a string and you can't use external_id: Use the search in app API call to search for the string.