Go to /data/db/ and open itemdb.xml. In there you'll find the definitions for all the items in the game, and it includes things like their inventory icons and filenames.
In some cases you can find an item by searching for the important part of the name ("Salvation" for the bow), but in otbher cases the engrish name is too different. So if you don't know or can't find the item ID, you can go to /data/local/xml/ and find item.english.txt and search for the english name there. Note the number at the beginning of the line. Then go back to itemdb.xml and search for...
.12345]
where 12345 is the number you saw on the line.