I recently endured the struggle to source an OEM spec battery at a minimal price and it was tough. You can source the Varta e44 on ebay for a very competitive rate but I was told that the reason why they're relatively cheap is because they won't offer any warranty with them.
The Bosch S5 007 from EuroCarparts is £170ish but that'll come with the warranty. As it turns out, the retailer who sells you the battery will have the warranty agreement with the manufacturer which they extend onto you. Hence the price difference.
A local parts store in East London was able to sort me out with the S5 for £110 with the 5 year warranty. Was a decent deal, slightly out of my budget but I scraped a few pennies together for it, the Warranty is whats gold in these things.
Hope that helps