or ebay seller
eBay.com.sg: Citric Acid Descaler for Avent Steam Steriliser (item 330326736439 end time May 08, 2009 06:38:44 SGT)
i remember i read about getting some lime juice to do the job. not sure how true.
i found this on the web.
If you have no Citric acid you can make up this solution, of 100ml vinegar mixed with 200ml cold water pour into steriliser. Allow to stand in the unit until any lime scale has dissolved.