Is there such a thing as a bone cleaver? When I was growing up, my mother had a massive, MASSIVE cleaver that she kept in the kitchen. It was immense and heavy. My father had gotten it when he worked in a butchery. This cleaver has since been lost to us. I would like to find something similar. When I asked her what the specific type of knife was, she said it was a "bone cleaver." It was easily 60 years old when I was little 30 years ago.

For the life of me, I can't find references anywhere to bone cleavers. I can find meat cleavers (but most have warnings not to use them for bone). They're smaller than this was, and also lighter. I can also find references to vegetable cleavers and Asian style cleavers. Again, they're smaller and lighter than this. They're hammers compared to the sledgehammer this thing was.

My mom used to use it expressly for cutting through bone. You'd raise it up, give it a little force to accompany it's natural weight, and it could go through almost anything with relative ease (up to and including femurs).

I know bone saws exist, and usually I just keep a spare blade around for my hacksaw to cut through bone when I break down a carcass. So, is there such a thing as a bone cleaver? Would it be a special order thing these days? I'm wondering if it may have been a custom made knife back then because I have never seen anything else like it.
Absolutely.

I have this one here: J.A. Henckels International Classic 6-inch Cleaver   ... it is billed as being designed "for chopping through joints and bones". These cleavers tend to have good, solid weight and a short blade bevel to give it endurance and power. You would not slice things with this. This is a momentum tool to crash through the target tissues with a confident swing. The hacksaw that you mention offers better control, in general.

The opposite of this is an Asian style cleaver that is used primarily for vegetables. Lighter, faster blade with a deeper bevel which makes it sharper for slicing but it would wear down faster under the impact of bones/joints.