No matter what, there is always the black market. If you ban guns, someone who really wants a gun will just get it off the black market. I remember hearing stories about people in the USSR (this was in the middle of the cold war, my uncle told me a story about a friend who used to run contraband into the USSR) paying huge sums of money just for a pair of designer American jeans.
Simply banning something does not stop people from getting their hands on it. Look at drugs, they are everywhere in some schools. They were band, but anyone that wants to smoke some pot just has to talk to the right people.
My point is banning something won't do anything really. I'm more worried about terrorists stealing highly radioactive materials rather than making it themselves. Those old Coleman kerosene lamps used to use D.U. in the ceramic burner. A teacher put a geiger counter next to an old one, and it through a lot of clicks. Even from a good 3' (1 meter) away, there were still a lot of clicks from the geiger counter. There are tons of materials in your home, under your kitchen sink even, that you can turn into a bomb, incendiary device, or something else dangerous that could kill people.
For anyone to make a dirty bomb that was really successful in causing long term economic problems, they would need something highly radioactive. Stealing nuclear waste from a nuclear power plant would probably be much easier in cheaper. Well maybe not easier in the U.S., but places were security and regulation is more relaxed.