Is it possible to buy 'ethical fur'?
Rework an antique fur coat!

My grandmother keeps encouraging me to take hers, which I really don't want -- but the point is there may be many other such coats that used to be valued and sought after, but are now of very little interest to most potential wearers. Asking around thrift stores or estate sale agents may give some leads; even if they would typically discard such items due to probable disinterest, letting them know you're interested means they could inform you next time an old coat is available.

If it's significantly damaged (moths, mold, just old age), all the better: it is unlikely to have any further use as a coat, but could readily be repurposed into accents on a costume. This is perhaps the most ethical use of all: you're not resulting in any new animals dying (even if humanely raised and slaughtered), but instead salvaging something that would otherwise end up in a landfill.

(But like anything ethical, it's sort of down to your personal perspective and comfort level. There's also an argument to be made that if it wasn't humane and ethical in the first place, it isn't now just because decades passed between the animal's demise and your use of its pelt.)