You can go to
ESF-World for that. Simple look in the right and click on "Models", and then "Beta v1.2 models" or something related to that. And installing them is rather easy.
Now, go to "steam\steamapps\yourname\Half-Life\ESF\Models\players\"
You will see loads of folders, now. If you want to change your current model lets say "ssjgoku". (That's Super Saiyan Goku) go to that folder. You will see "ssjgoku.mdl". Remove that file, and put the ssj3goku.mdl in the folder. Once you have done that, and this is really
important. Rename the ssj3goku.mdl to ssjgoku.mdl.
why you may ask? Otherwise ESF will give you an error that the model cannot be found. The model file name should be exactly the same as the original. If it already exists, simple overwrite it.