Gonna be quite challenging to get "1/2 cup of rendered sheep fat"

In Australia where lamb is more common than in the US, it'd be pretty easy to keep the fat after roasting a lamb shoulder, or to freeze the fatty offcuts from forequarter chops and render them after I had enough.

Lamb is quite popular down here in Australia. I usually keep some rendered fat from my roasts etc. Sometimes I cook other meats with it and it adds heaps of flavour. I do that with pork fat as well.

Lamb is usually the red meat of choice, other than pork, almost everywhere besides the US.

I was able to get some tail fat from a halal butcher. But never seen lamb fat anywhere else

if you have a good butcher they might be able to get you some that you can render, but its a hassle. I've done this for beef fat to make some british pie things.