Courtesy of the Neural Engineering Lab at CCNY, the BONSIA and SPHERES free modeling tools are live and FREE. These modeling packages can be used to help design TES montages including for tDCS, tACS, tRNS, and more. 

Here is the paper that explains the principles behind these tools, Clinician accessible tools for GUI computational models of transcranial electrical stimulation: BONSAI and SPHERES. 

Bonsai Modeling Software – web based modeling tool

Spheres Modeling Software – requires Matlab

Additional Details can be found by referencing this paper, The Point Spread Function of the Human Head and its Implications for Transcranial Current Stimulation