The best way is to go to a championship show, where you are spoilt for choice and buy a lead that suits you. Most IS exhibitors use brown slip leads, either leather or fabric. Choose one of good length that is not too thick but thick enough not to break. If in doubt ask around.
Your comment made me laugh Wilko........I too have had bitten leads!
I used to use fabric leads but they cut into my hands so I invested in a (at the time) very expensive soft leather slip line...left it hanging over the bench divider & my dog chewed it to pieces....I only ever got to use it once!!!
Setters.......gotta love em huh!
I can testify that your leads do last, I bought a round one from you in Dortmund years ago & it is still as good as new. :))