Soldato
I had a good look around, but other than very pricey ones (£30+) I didn't see anywhere that I trusted to send the genuine article.
Check out this article about "compatible"/higher capacity mobile batteries as well:
http://batteryboss.org/
I just rang Samsung. They passed me on to some other place. I rang them and they gave me a phone number with a different code which happened to be the same place as the first number I called. They then gave me another number of a samsung spares place. Rang them and they told me they don't do samsung batteries. For the love of christ, samsung don't exactly make it easy to buy a spare battery. I rang chitterchatter where I bought my phone and they don't sell batteries.