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.One reason that it's so hard to refine the small amount of U-235 out of the larger mass of U-238 is because it's *all the same element* -- it's all Uranium (just two different isotopes of it). This means that most normal chemical extraction processes, which are so effective in other types of extractions, quite simply will not work. They have no way of differentiating one "flavor" of Uranium from the other (since they both have identical electron shells, which is the "chemical signature" of an element and what determines its chemical properties and behavior).
So you have to get a bit more tricky to separate the two. One method is to vaporize the mix, then use a centrifuge. This causes the (very slightly) heavier U-238 atoms to deflect farther outward than the lighter U-235 atoms, and causes the mixture to separate a bit. But this has to be repeated many, many, many times to eliminate most of the U-235. It's not an easy task, or one that can be hidden in a garage.
The plutonium used in bombs is Pu-239. It doesn't occur naturally in any meaningful sense;The reason is that it has a half-life on the order of tens of thousands of years. Thus, any which might have been present during the formation of the Earth has already decayed away to practically nothing. U-235 is longer lasting (half-life of millions of years), and thus although much of it has decayed since the Earth was formed, there's still quite a bit left in the ground.
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.It's easier to extract because it's chemically different from U-235, and thus normal chemical processes can be used to "grab" one element and wash it out of the other.
On the other hand, since it is much more radioactive, the extraction process is that much more "messy" -- you need greater radiation protection, and the "leftovers" are a bigger problem to deal with.
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.This is because the P-239 "wants" to react more vigorously, and thus as you try to bring a critical mass together, it quickly starts to react, and that reaction tends to push the pieces back apart. You can often get a fizzle instead of a big boom. It takes a higher-tech design and implosion timing to make the P-239 slam together precisely enough and fast enough that it doesn't have a chance to "fizzle" itself before it goes critical enough to make a large explosion.