Several animalics--castoreum, ambergris, musk, civet, and hyraceum--were once used in perfumes. Nowadays, many have been replaced by synthetics, none of which match the real thing. While the ethics of using castoreum is somewhat ambiguous--it's really a byproduct of trapping beavers, not the main incentive as it is for musk--it is still commonly used in perfumes.
Castoreum is usually sold as the dried gland. Since it can be difficult to find, I had to think of where I could track it down and it occurred to me that trappers probably have it to attract beavers. Indeed they do and sell it as a thick black paste. Having heard about its strong aroma, I opened the jar in the garden, but found its aroma surprisingly gentle and clearly defined.
To produce the castoreum used in perfumery, the gland or the paste must be tinctured in ethanol (drinking alcohol). It tinctures rather quickly--in a week or so--and throws off a thick sediment. I carefully decant off the liquid with a pipette into a clean bottle.
When I think castoreum, I think leather, tobacco, fossilized amber, and saffron--a perfect base for a leather perfume. Its use, however, is not limited to leather complexes, and it is used in many creations including many famous ones from which it has now been eliminated. I know of no replacement for castoreum.