Okay, so buying them is right out. But wait, don't you acquire them through playing Ranked Battles? Yes. Yes, you do. Here's when you get them and how many:
Well that seems rather generous. That's a whopping 150 Juliet Charlie signals in a season of Ranked Battles. At least, it might seem generous until you look at how many battles people are playing to get anywhere. I mean, let's take a look at the top Ranked players on the North American server as I'm writing this (which is 14:00 Eastern), and they're averaging 169 games each and half of them are still stuck at Rank 5. n3v3rforgott3n, mad props for being the first to reach Rank One on the North American server, btw! So yeah, generous as it may appear, you categorically will not earn enough signals through Ranked Battles to cover your matches.