I have a bottle of Ambra that has been completely neglected since I discovered BPAL, it used to be one of my favorite fragrances.
Of the BPAL i've tried so far I haven't come across one that smells the same, but for what it's worth here are some similarities I've found:
Cathedral has a similar woody note, Snake Oil a similar sweetness. Penitence as a similar overall impression of Ambra to me, but a little less sweet. Sloth also has the "feel" of Ambra but spicier.
Now I have Ambra on one arm and Penitence on the other and I would say they are the closest to each other I can find.