In the US, I would be very careful with feral cats as they can carry rabies. If you want them out from under your house then you need to find a vet who will treat ferals because its a risky business and not all of them will do it. They will need to be trapped, checked for rabies and vaccinated and neutered before you can release them again. When I lived in New Jersey, the brand of humane traps was called Have A Hart but they are not cheap. If you can find a vet who treats ferals or a cat charity, they may be able to loan or rent you a trap and give you help trapping and treating the cats. Personally I would leave them alone unless you have got family pets and then I would trap and remove the ferals for the health and safety of your family pets.