I usually measure max battery stowage space and buy the largest capacity that will fit snugly within. Try to match terminal orientation and type though. Recommend a decent brand but waste of time going for exotic ones that take ages to expire at three times the cost. Batteries are consumables so better to have one that isn't a decade or more old. Aim for 55 amp hour minimum capacity on a trad.
Battery Megastore and Tayna are both excellent mail order sources.
