Merc battery

Usually your local motor factor. There's nothing special about the battery, but delivery is prohibitive for online sales - couriers won't touch them. You should be able to get one for around £40 (that's what I paid for a Previa battery which is about the same size - I own both cars). I had the same price quoted by County Batteries and Autospares/Sutton Autofactors (the physical branch of buypartsby), but the latter had the longer warranty.

Costco are usually great value if you know a member