Why are fissionable elements such as U-235 so hard to come by? Is there a finite quanity on earth? Can they be manufactured or refined?
I think the real deal is China and the former commie states, versus the free world... using "terrorist" events to trigger another world war.
Size matters. Ours is bigger.
There's about five million tons of it in places where it could be economically mined, so availability isn't the problem—it's not nearly as rare as silver, for instance. Refining away contaminants such as chlorine and oxygen is also no problem—people have been doing that since 1841.
The problem is that there are basically two kinds of uranium, U-235 and U-238. The two are found mixed together, 99.3% U-238 to 0.7% U-235. U-238 is not sufficiently fissionable to be used for reactors or bombs, so the task is to "enrich" the uranium by removing U-238 atoms. (The U-238 waste material left over is what's called "depleted uranium.") Normal uranium is 0.7% U-235, as mentioned; "low-grade" uranium is about 3%, "high-grade" about 20%, and "weapons-grade" above 80%. From these numbers, you can see how much refinement is required, work that requires very advanced chemical engineering skills. For all but the most technologically advanced countries, it is much easier to acquire U-235 than to produce it.
The plutonium used in bombs is Pu-239. It doesn't occur naturally in any meaningful sense; it must be manufactured by exposing U-238 to slow neutrons. This happens all the time in nuclear reactors, however, so plutonium is surprisingly easy to come by. There's about 1500 tons of plutonium in the world, 80% of it existing in the form of nuclear waste from civilian reactors.
Extracting Pu-239 from reactor waste is much, much simpler than enriching U-235, and since Pu-239 is much more radioactive than U-235, less of it is required for building bombs. However, precisely because it is more radioactive, it requires more sophisticated bomb design—the large, crude gun-type "Fat Man" bomb dropped on Hiroshima used U-235, while to use plutonium required the smaller, more advanced implosion-type bombs tested at Alamogordo and dropped on Nagasaki.
So countries like Iraq and North Korea have a tough choice: Either they invest in very expensive, very difficult and very traceable U-235 enrichment facilities, or they have to develop complicated bombs that require very high-tech and hard-to-acquire components. Iraq chose the former route, and got its facility bombed for the effort; now Iraq is falling back to the easier-to-produce plutonium. North Korea chose the latter route, and while it now has lots of plutonium, it's having trouble developing an actual bomb to use it; so North Korea is rapidly developing uranium enrichment technology.
If there were a source of weapons-grade U-235 in the world, we'd all be in a lot of trouble. However, weapons-grade U-235 is only used for certain specialized components of advanced thermonuclear warheads, so there simply aren't large quantities of it lying around to be stolen or bought. Any "nuclear bomb material" you read about being acquired by terrorists is almost certainly Pu-239 or highly-enriched U-235, not weapons-grade U-235. Nasty stuff, but no nastier than jet fuel proved to be.