If they had waited another month, they could probably (get) a better house.

1. This is a third conditional sentence, referring to an unreal past situation and its probable result.
2. The structure is 'If + past perfect, ... would have + past participle'.
3. However, the sentence uses 'could' instead of 'would have', indicating a possibility rather than a certainty. The structure is 'If + past perfect, ... could + base verb'.
4. Therefore, the correct form is 'get'.