I think one way is to observe a particle in superposition – that’s going to be random by default. There’s even an app called “Universe Splitter” on iOS that allows you to perform a measurement on a particle in superposition and based on its outcome do a particular action which is guaranteed to be random.
I am not exactly sure how does the Rule 30 Cellular Automata is random – I mean, sure, the outcome looks random, but how do you choose a subset of that result set? Isn’t the decision of choosing going to be determined? Isn’t it the same as using a random number generator with a seed? Or is that actually the point, that’s what a random number generator uses under the hood?