1 昨天他不来，今天他再不来，我们去找他吧。 这个句子是最恰当的。“再” 表示“will happen again, has not happened"，比如：”我要再去一次北京“, which mean I will go to Beijing again. So here "不来" has not happened, if it happen again today, we go to find him. 2 ”还“ is more causal, and if you say "昨天他没来，今天他还不来” that means he is still not come today, it is already know the result is "not coming". So if fit your sentence, should be"昨天他没来，今天他要是还不来，我们就去找他“，here ”要是....就“ is same to "如果....就”，which is also more causal. 3 也/还 They can both mean happen again, and "还“ will more emphasis "still". And the thing has already happened. So if you say "昨天他没来，今天也不会来" means he will not come today,too, so it has no "if" meaning in it. And "昨天他没来，今天还是不会来" will be more natural, and emphasis "still will not", also mean he will not come today. So "也/还不会” is not fit your sentence.

再 = again 也 = also 還 = still. All these words will work in your sentence. However, the nuances will be different.

昨天他没来. (period) 如果他今天也不来，我们就去找他吧！

昨天他没来，今天他再不来，我们去找他吧！ 昨天他没来，今天他还不来，我们去找他吧！ Both sentences work. The latter one sounds more causal to me while the first one is more formal. 或者： 昨天他没来，今天也/还不会来，我们去找他吧！ This one doesn't fit your sentence very well so I wouldn't use it. I don't see a way to tweak this one to fit your need. Maybe someone else can :) My translation for your sentence: Yesterday he didn't come, if he doesn't come again today, we'll go look for him. 昨天他没来。 如果他今天再不来， 我们就去找他。