Get Item by String?

For the function
player.GiveItem(Item item);
I need a Item variable.
How do I get a Item variable by a String shortname? for example (pseudo code)

Code:

Item ak = GetItemByString("rifle_ak");
player.GiveItem(ak);

or maybe something like

Code:

Item ak = new Item("rifle_ak");
player.GiveItem(ak);

 

Item ak = ItemManager.CreateByName(item_short_name, count);