Nerf-Flip Clip Refill Pack - £5 Delivered @ Debenhams + possible Quidco/TCB.
Comes with 30 Darts. 36 darts on their own normally cost £5.
£10.39 at Play.com and Amazon.
Great fun for extended Nerf wars. Includes 2 cartridges and 30 darts, so as 1 cartridge runs out, pull it out, flip it over and continue shooting.