Easiest way to get a random number is to start with some input (maybe a counter?) and hash it. And there you go, you have several bits of randomness right there. This is what I've had in my projects like dojo-shooter to, say, pick the coordinates where a zombie spawns.
This works fine for trivial games but I can easily predict exactly where the zombies will spawn well before the game starts.
You can use randomness from other contracts as salt. But idea is pretty much the same.