The icons are several manila files. many cabs and themes available are only those icons so if you find some you like you're good that way, just be sure to read what changes with each. Also if you find the ones you want and they are not in cab form you can use total commander or resco explorer to drag/drop them into your windows folder and overwrite the originals (works the same) be sure to turn off tf3d.
Also, if you're wanting to create your own first go read the thread in the themes forum about customizing tf3d... here:
http://forum.ppcgeeks.com/forumdisplay.php?f=75
That should get you started.
Also, everyone here is pretty helpful.. check out the "post your home screen" thread and if you find some icons you like just put in a request, most of us will post them up for you to use!