Looks like it's also known as mopane (https://en.wikipedia.org/wiki/Mopane). The ads I saw for use in aquariums look like a pretty gnarly wood for pen turning. Dense enough to sink and use as underwater habitat in aquariums. Looks like it may need to be cast after turning it down some, or filled once the turning gets close.
I've seen pieces of this in pet stores, specifically PetSmart and Petco, that would be large enough to cut a blank or two from. Many, as was previously said, can be pretty gnarly, so you'd have to shop around a bit for size and shape. My step dad was able to find a piece at Petco that should yeild at least two decent blanks.