In the UK, an equivalent is available from http://www.smallbattery.company.org.uk/ Search for a 523 -- the info pages will confirm that you have found the right one. It is quite expensive, so I would check oher battery specialists too.

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I like it how the description page actually says that the battery is used by the Plus, 512K and 512KE - they obviously know their stuff. It is expensive though, given the current fairly low value of Pluses.