Where to buy Nikon D5100 extras?

I just got a Nikon D5100 and I'm lookig to buy new lenses but places I've been to seem kinda expensive. Where would I buy good lenses to would work for the D5100 at a good affordable price. Also I'm looking for the battery pack that goes under the camera but forgot what it's called. Any advice of what to buy and what not buy would be great!

With quality comes cost. Nikon doesn't sell anything cheap. Try a site like amazon.

Amazon would be a good place to start looking. There are aftermarket brand lenses and other goodies for the Nikon DSLR camera. For lenses you can try Sigma, Tokina and Tamron.

Any batch of lenses? Or are you looking to accomplish something specific.

The battery pack is called a grip.