Are you getting indicator lights? Does it charge? Any signs of life when you press the power button? If you get nothing at all, it's a good idea to discharge the power circuit. Remove the battery and AC cord. Hold down the power button for 10-30 seconds. Without letting go, reattach the AC cord and continue holding power for another 10-30 seconds. Release the button and try turning it on normally. (On Windows machines, this is also a good way to clear the CMOS on some machines.)
I've found Batteries Plus to be a good source for laptop and mobile batteries. Most are brand names like Duracell. They do sell batteries for Apple devices. Looks like the iPhone batteries are Rayovac.
Nick, disconnecting the battery is precautionary and highly recommended. Even the slightest damage to the battery while working around it can cause a thermal event. It’s best to just get it out of the way altogether.