In the US, they can be had from atlantahobby.com
http://www.atlantahobby.com/shopexd.asp?id=6220
If ordering from duralite, they told me they can get anything that flightpower makes, they just have to order it. They said they place orders weekly for products from flightpower.
You could also go with the Trakpower packs if you don't mind wiring them to get 4S.