https://www.amazon.com/Everything-need-about-Collar-Training/dp/1521126550/ref=sr_1_1?dchild=1&gclid=EAIaIQobChMI6Ovtns6W7AIVEtvACh0jWQbHEAAYASAAEgLq1vD_BwE&hvadid=397153423619&hvdev=c&hvlocphy=9014680&hvnetw=g&hvqmt=b&hvrand=3664938269199274610&hvtargid=kwd-843943529728&hydadcr=28985_9847338&keywords=e+collar+larry+krohn&qid=1601665849&sr=8-1&tag=googhydr-20
http://www.pakmasters.com