There are two types of battery packs that you can get:

The first type is your readily available manufacturer packs. These are specific to flashes, as different flash manufacturers use different high voltage power sockets. (To keep you locked into the ecosystem and make you spend more money). The two most common type of plugs are Nikon and Canons three pin plugs. Which both offer faster flash recharge speeds and a higher capacity of possible flashes.

Your second type is your homebrew DIY option which uses Dummy AA batteries in order to connect bigger batteries, to a flash. The only benefit to these battery packs is the higher capacity of possible flashes that they provide. An example can be found here: http://www.diyphotography.net/the-external-flash-power-that-will-last-till-hell-freezes/

In terms of buying a manufacture battery pack, you don't have to limit it to buying Nikon's battery packs, you can also buy third party battery packs by manufacturers such as Godox and Yongnuo. Just ensure that you buy the correct cables / and or versions for use with your flashes. There is no benefit to staying with Nikon and buying there battery pack, at the end of the day all battery packs do the same regardless of who they are manufactured by. You'll just end up paying more money. Examples of these here: http://pages.ebay.com/link/?nav=item.view&id=321512496826&alt=web http://pages.ebay.com/link/?nav=item.view&id=151021626385&alt=web

Hopefully that answers your question :)