I have no idea whether the RCA 40468A FET is still available or not. It came in a 4 pin metal can - with the 4th pin connected to the substrate. I have successfully substituted other N-channel devices, such as the HEP-802 - which comes in a plastic TO-92 package. Simply ignore the substrate connection. Other good quality N-channel FET`s can probably be used as well. Both articles stressed handling precautions for FET`s.