According to ATMA, normal Diablo has the best chances to drop it - 1 in 4300. Among the regular monsters, a dark familiar in normal City of the Damned has the highest chance - about 1 in 3 million. Normal cows are neary as good and as they are very easy to kill and appear en masse, that might be the best option. BTW, MF increases the chance to drop set items by (MF*500)/(MF+500).
Gambling for leather boots with a L32 character (5 levels higher than the qlvl of these boots) provides a rather good chance for them. A small percentage of the boots might become exceptional (I'm not sure about that), but among the normal ones, 1 in 2000 will be a set item - Tancred's Hobnails.
BTW, I found these boots 3-4 times in the last season and sometimes people put them up for trade, so you can as well wait until they finally drop.