I agree with Famine about losing darts. The three darts that originally went with my Secret Shot have all been absorbed into the clutter of my friends' houses.
Let's just say sarcasm is my specialty as far as suggestions for nerf goes. In all honesty, I don't really see the draw in having a surfeit of darts in a Nerf skirmish. Limited ammo means expanded thinking.
The darts problem is easy, actually. You work out what kinds of guns people will be bringing a few days in advance. Assuming you have the skillz, get the local hobby shop or hardware store to give you a discount for the chance (yeah, right) to be advertised or set up a booth for the war. There are loads of people dying to turn a buck honestly by selling stuff to kids (Ok, maybe not that honest.) With your few days in advance, you make enough darts to at least justify selling them.