If he’s using it he probably just has a prescription or a doctor that provides it to him. Ketamine isn’t all that hard to get prescribed and it is, frankly, a pretty mellow drug.

In developed countries it's not quite that straightforward, but "his" plane will be owned by Totally Not Musk LLC, not Musk himself, so any putative drugs on it are not his problem.

In developing countries, anything goes. I've paid $15 for "VIP" clearance in Indonesia, which means I sit in a lounge sipping tea and my passport comes back to me stamped without me ever seeing an immigration official.

Ketamine is diverted from official sources, usually where it is produced(India, Denmark are common). It's not worth it to make in a lab given how plentiful it is. Its everywhere, in every city. He probably has a guy that gets it online. You can easily buy ounces on darknet markets. I don't recommend you do this-- I just find it interesting how this stuff is all going on as a second blackmarket economy in absolutely huge quantities.

in 2019, the FDA approved a self-administrated esketamine nasal spray. it's available by prescription.