Rumblers are not all available from the get go, they will be available to be unlocked as you progress through the Leagues.
Note: when you match against a higher level player, there is a chance you might be playing in a higher level League, as that will be the one your opponent progressed to. Winning in a higher level League, will also award you with Chests (and loot) from that League: more gold, more gems, and Rumblers from that League that would normally not be available yet.