You can find the item tag by going into the toolset and looking through the relevant area for the item which can be done in a few ways.
I'd do it you, but I don't have the game installed currently - so you'd either have to rely on others or find it yourself.
It isn't terrible difficult finding the prober tag for the item, so give it a shot
Once you find it you can use the dm_spawnitem (I think the command is) to spawn in the item, if you are still in the prober chapter.