Though I don't have a Spotmatic, having looked into it a bit previously, my understanding is that while the Spotmatics originally used a mercury battery, because they used either a balanced bridge or dual ammeter circuit, a mercury battery is not a necessity, as they are not sensitive to input voltage.
The original Spotmatic should use a 387 size silver oxide watch mattery, while the F should use an SR44 silver oxide battery (though may need an o-ring to center it in the compartment). I would stick to silver oxide as while alkaline batteries can also be found in these sizes, the silver oxide batteries will have a longer life, flatter discharge curve over their life and are much less prone to leakage.