Those companies used to sell external battery chargers (Dr.Bott's side bar still has a battery charger category but there's nothing in it) but there's almost no market for them. Look online for wallstreet battery chargers, they almost have to give them away to get rid of the stock. Since the G3 Powerbooks I think there's only been one external battery charger since then and that's at MCE and only for iBooks. Couple of those on eBay but other than that its all older battery chargers

Battery life is far better now, so there's less of a need for more than one battery. One top of that do you want to pay $250+ to get a second battery and a battery cradle or $70 to get a car adapter and a spare AC adapter - that in the end will be smaller and lighter?