Is there a cheat to get omanyte and kabuto without trading in leaf green?

There is. First, you have to use the master code D8BAE4D9 4864DCE5

A86CDBA5 19BA49B3. If that one does not work, you can also try the master code 0000BE99000A 1003DAE60007. Then you have to use this code: 82003884 0165 (if you need the Helix fossil) or 82003884 0166 (Dome fossil). Then you can buy it at your closest PokeMart for free. It will be the first option. Good luck :D