this is one i got when i first got car
http://www.carparts4less.co.uk/cp4l/c/BMW_Z4+Coupe_3.0_2007/p/car-parts/car-electrics-and-car-lighting/electrical/car-battery/?444771108&1&a2f97f73d48ee3e266caceef38c26e48463358f6&000020
you can find a discount code 15 quid off if you spend over 100.
had similar battery in old car for 12 years, always fired up on daily use, wasnt till this year when it was in minus temps it struggled and once needed a jump.