You would be horrified at the weapons that are available on the open market. There are place, generally 3rd World, but not always, that you can get everything up to and including heavy armor for cash on the barrel.
What prevents this is that the bubble around the 1st Family tends to be much more robust than is imagined. Getting something like an RPG or an ATGM into effective engagement range is extraordinarily difficult.
This sort of scenario might be difficult for a domestic group to arrange (although AT-4 do walk away from NG armories from time to time), but it could be done. It would be easier for an international group, or for a domestic one with connections to either radicals or organized crime.
It is much more of a logistical challenge than the easier to obtain, although far less selective, truck bomb.
There is a reason that the Chief of Detail tends to get very little sleep.
I think this sort of thing would probably be easier to pull off in a foreign country. Some more than others.