Here is a good article on the use of ivomec Eprinex (it is the only ivermectin product I would recommend) you can order it from several places online
http://shilala.homestead.com/ivomec.html
It is still a good idea to oil the legs even when you have used the ivermectin.
Don't forget to THOROUGHLY clean out and treat your coop... reinfestation from their coop/nestboxes is often overlooked in the management of parasites.
Here is another instruction page for using ivomec... (giving too little will only encourage resistance issues > it is one of the safest wormers )
http://shilala.homestead.com/ivomec.html