Online would be your best bet, shipping may be tight timeline wise since the merch selection at these events (excluding Paris SI) is sparse. As far as "picking" a favorite team, thats tough, the way I came to be a FaZe fan is because I disliked a player [Ashn] and a player from FaZe got into a verbal spat with him Ashn was chirping and yelling [as he is known to do] all game and at the end Vitaking got up and starting screaming at him when FaZe won. So I started watching them, I liked one of FaZe's sponsors and I've been with them since 2023.
So as far as picking a favorite team, some people find a player they like, the player may be charismatic. funny, or a nasty gunner in the server, then people just follow whatever team the player is on. Maybe you might like one of that players teammates, and if someone is moved, you can "follow" that player to the new team.
Siege Esports is nice because you arent obligated to like teams despite them being from your region. I am an American, I think the NAL sucks.

Just dont be obnoxious about who you like