It's a typical issue in many autopilots... You can get an external GPS+Compass, as the one we have at our store.
You could check the schematics and go digging. But:
1) If you want to feed some from the 5v pins, it has a PTC, the nominal hold current is about 350mA
2) If you are asking how many mA you can drag from each PWM singal, about 25mA if I'm not wrong.