I have attached for you here the Expression Node (a unitypackage). Please note that this is only a beta/early version of the node.
Thank you for the extra information on the Probability node. I will implement such a node in FlowCanvas in the next few days and will also be there in the next version. While you can use NodeCanvas Probability Selector, from what I see you would prefer using FlowCanvas (with the new Probability Node when it’s ready) though :).
Let me know what you think regarding the attached Expression Node by the way.