Depending on the brand of rod set, you can buy replacement rods from some suppliers as single items. However, some replacement rods are only available in multiples of two or five.
You can buy a rod set spares kit and the contents usually includes the following –
Magnet
Gender changer
Mini eye and ring
Domed bullet
Flat bullet
An individual replacement battery for the beam attachment of a rod set is available, from the supplier.
Multiple packs of 1.5v, AG3 batteries for the beam attachment are available from online merchandisers.