  Pacemaker batteries. 
These batteries are designed to be implanted in patients to power their cardiac pacemakers.  The seller reports they are from the liquidation of a biomedical products company and had an original list price of $500 each.  Jeez, for that price I'll slice my own chest open and drop in a couple of Duracells.  They are probably single-use lithium batteries, but maybe they are lithium-ion rechargeable, I'm not sure.  No, I won't sell you one for your pacemaker. 
Note that these are, from a safety point of view, probably superior to the one I have listed under plutonium. 
Source: eBay seller heruur 
Contributor: Theodore Gray 
Acquired: 30 December, 2003 
Text Updated: 11 March, 2007 
Price: $14.50 
Size: 2.5" 
Purity: <5% 
Sample Group: Medical
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