If you've ever wanted to upgrade the voltage in your Barricade, Rayven, Stryfe, or any battery-driven foam blaster, now might be a great time to cash in! Amazon.com has a sale on UltraFire 14500 1200mah 3.6V Rechargeable Li-Ion Batteries. These fellas are excellent for voltage modifications. With this set, you get 2 Ultrafire Batteries and a charger for them. They're usually 16 bucks, but there's a sale on 'em for $13.98 from Amazon.com. While these might seem too expensive for some, because they're rechargeable, these batteries can be used over and over again. They also hold a charge for quite a long time.
Still out of your budget? Try picking up Energizer Ultimate Lithium batteries at any major retailer. They're still a little more expensive than normal batteries, but they last longer and have a slightly higher voltage than normal AA batteries. You'll still get a bit of a power boost out of your blaster... just not as extreme of a difference that you would get from Ultrafires.