There's another problem: Dirty bombs are far more dangerous to their makers than to those exposed to them. There are two basic threat modes: environmental and ingested. The environmental threat is basically nil:
To have a 50% chance of killing someone with your bomb you need to expose them to 5 sieverts of radiation. Neither alpha nor beta radiation pose any real threat outside the body, so this must be in the form of gamma radiation. Note that gamma radiation is hard to shield against, bombmakers in a garage aren't going to be able to protect themselves. Lets suppose they're suicide bombers, they don't care about 5 sieverts. However, 10x this dose will promptly incapacitate them. (Probably less, we don't have good data. Very few people have been exposed to such doses and figuring out how much they were exposed to is difficult.) Lets build a bomb that can contaminate a football field to 5 sieverts/hr. (This will only kill those that hang around after the boom!) We put it in a van.
Our target is 360' x 160' = 57,600 ft^2. The first hit off Google for the size of a cargo van gives us about 45 ft^2 in back. Thus the radiation coming off the as it sits there is more than 1000x the radiation at the target. Lets say 1000x. It's going to be radiating at 83 sieverts/minute. How does your terrorist do anything but keel over?
Realistically, all that external exposure can do is contaminate an area and force a cleanup.
If you actually want harm you need to go with the isotopes that the body grabs.
One of the scary ones from nuclear incidents is iodine-131. The body sticks it in the thyroid, the concentration isn't a good thing. However, it has a half life of 8 days. Where are your dirty bombers going to get it??? The only supplies are a small amount the nuclear medicine guys use--and you can't stockpile it. Forget this one.
The other two are caesium-137 and strontium-90. These are the only two things they'll be able to use--and in both cases the effect is an increased cancer risk, not immediate death. What's the point of a terrorist attack where nobody dies?