Lewis,
First of all, the Nephilims were not half angel half humans as some believe them to be. Simply because angels were not created with reproductive capabilities as humans are. Jesus tells us in Matthew 22:30 that angels do not engage in sexual activities, at least not the ones in heaven anyway. This does not mean however that evil angels on earth could have used the bodies of ungodly men, by demonic possession, to achieve their evil purpose of producing an evil generation of people as Genesis 6:12 tells us.
There are more reasonable suggestions as to who the Nephilims of Noahs time were as H.C. Leupold a Bible scholar believes that the sons of God were descendants of Seth, the godly line described in Genesis 4:25-5:32. Leupold states: "But who were these 'sons of God'? Without a shadow of a doubt, the Sethites." H.C. Leupold, Exposition of Genesis, Vol. 1 (Ann Arbor, MI: Wartburg Press, 1942), p. 250.
Sorry, don't know of any extra biblical references to angels or demons that does not depart from the word of God.