You could look on eBay and Bricklink for used packs, and also for the transformers.
The dark bluish gray battery pack
requires the AC transformer
The white battery pack
requires the DC transformer
The white battery pack has a higher capacity, so you don't have to charge it as often (I think it's about 2000 mah vs. 1400 mah).
You can charge them while in the NXT, so just build your robot so that you physically have access to the charging port.
The battery pack sticks down by 1L compared to the AA battery cover, so keep that in mind when building.
I'm all for gun control... that's why I use both hands when shooting