A 7.4v LiPo will provide more than enough juice for your Pulse Rifle.
I squeezed an 11.1v in mine, just for a more realistic (read insane) rate of fire.
There are many places to get them from. Over here in the UK, I use
http://www.component-shop.co.uk Remember it will need it's own special charger (about £10) and to make sure you don't destroy the battery, you'll need an alarm (about £5).
I'm sure there are a myriad of Airsoft shops over in the states that could supply you with a LiPo battery, or if that's a bit of a step too far for you, then they will be able to supply a standard NiMh Stick style battery.
Redwolf, evike... I'm sure someone can mention many better places.
You will need either a 8.4v or I think you could get in a 9.6v one (this may need some internal modification), and go for a high mAh, like 1500 or 1600, which I think is the largest they currently do in small 2/3A cell types.
NiMh:
http://www.componentshop.co.uk/8-4v-160 ... -line.htmlhttp://www.componentshop.co.uk/9-6v-160 ... -line.htmlLiPo:
http://www.componentshop.co.uk/7-4v-130 ... -long.htmlhttp://www.componentshop.co.uk/7-4v-130 ... -long.html