Yo drop calcs are going to be your best friend because you’ll be amazed at the miniscule chances for virtually anything worthwhile and how much worse they are in the lower difficulties…
In short Lems are possible to drop in nightmare, best chance is hellforge but from general drops, it is much more likely you’ll find one in hell. Running hell countess is a decent spot…but I don’t believe in hunting for any specific item in this game, drop chances especially for runes are extemely small hence why we constantly see these “i farmed chaos/trav/countess/summoner/etc 46 times and nothing dropped!”
If you’re a pally you should be able to find a 4 os shield anytime from norm cows on. Otherwise for all other classes you have no choice but to use a monarch, an elite kite/dragon shield. Elite items can drop in a5 nm but are extremely rare. You can try to run a1 hell a bunch of times, run crypt/mausoleum/tower/cata and open all the armor racks you come across but I’ve been down that road and 99.999% only resulted in frustration.