get the cheapest batter sutable for your car online. they last forever if you use your car/condition them regualry.
if it it doesnt get used often it will be fooked in a few years regardless!
I still have a battery stamped last charged 1993 and its going strong in my 72 rrc after the rest of its life in a nissan primera used every day.
my £130 battery in my bmw which hardly ever got used was dead after 2 years and wouldnt hold charge/provide cranking amps even though it had a 4 year warrenty!