If the angels that fell with satan are in chains, where do we get demons?
Answer: The New Testament speaks of the operation of demons (devils) many times. So plainly there are demons who are not bound who do the work of Satan in the world. In Luke 8:31 the demons who possessed the man at Gadarenes and begged Christ not to send them "into the deep." That is were part of the fallen angels who rebelled against God are presently chained or kept. It is the same place that Satan will be bound in Revelation 20:1-3 during the Millennium call the bottomless pit.
Revelation 9:1-11 says demons in the form of hellish locust will be released out of the pit during the Tribulation. Further the Book of Revelation says that all demons will be bound in the pit during the Millennium. Rev. 20:1-3. Satan and all fallen angels (devils or demons) will be cast into Gehenna at the Great White Throne judgment of Revelation 20:10. The bound angels will be brought out of the Tartarus as 2 Peter 2:4 and Jude 6 state. So presently there are demons serving Satan who are present in the world and do the Devil's bidding.
Paul explains the assaults of Satan and is demons saying, "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places." (Ephesians 6:12)
Part of the host of fallen angels reside in the Tartarus for a time, but it seems they all will be released at Satan's final rebellion at the end of the Millennium.