Water tank anode rod replacement part
There are only a few types of anodes out there -- different materials (aluminum, magnesium, aluminum/zinc) and different form factors (rigid vs flexible/segmented, hex head vs combination).

Your heater comes stock with an aluminum hex anode (all American/Whirlpool/US Craftmaster heaters use aluminum, and the manual for your heater has a parts diagram on page 30 that depicts a hex anode -- so you should be able to replace it with a standard hex anode, unless you have overhead clearance problems that require a flexible unit.  I'd get a magnesium anode for your replacement if at all possible -- our bodies don't get along particularly well with aluminum ions.