Lithium batteries pack more power for the weight than other technologies, but they get expensive for large batteries. Although it has never happened to me, lithium batteries are notorious for catching fire if damaged or improperly charged.
You can pick up everything you need from stores that sell R/C airplane supplies. I would also check out on-line stores like Hobby King. Besides batteries, you'll be looking for chargers, motors, controllers and servos. Just about any of the motor controllers (ESC's) have a 5V circuit which can be used for powering the PI.
Lithium batteries for RC airplanes are categorized with 3 numbers -
- the capacity (in milliamp hours),
- the voltage (how many cells are stacked in series), and
- the max discharge rate (a ratio).
A battery described as "2200 3S 40" would provide 2200 milliamp-hours of power at roughly 11.1 V (3x3.7V) with a max discharge rate of 88 amps (40x2200).