Best way to power a 19 servo robot?

Other Hardware General Electronics
2

So that is 18 times 2.5 A (mental arithmetic - 36 plus 9 = 45 Amps) plus another 900 mA.

Sounds like a large RC airplane 2S pack with a minimum 50 Amp rating and a little switchmode regulator to tone it down to 6 V for the MG90.