There's been a discussion on here, about buying batteries of e-bay. The best place to look for parts like that and basic service parts, is on places like the eurocarparts site. They do batteries for our vans, from less than £40 up to £80 at the moment, to suit various budgets.
Recently, I found a brand new part unavailable in the UK, by putting the part number into Google. Found it from a supplier in France. So, my advice is, only go to e-bay as a last resort, for stuff that is not available from VW specific parts suppliers.
