I am no nuclear weapon expert, though I have read a lot of stuff that has been declassified in recent decades, and the obvious Q to ask is:
If it is so apparently easy to get one's hands on a bomb, why is the UK, America, and a few other places (like Israel) still standing?
The IRA would have absolutely loved to blow up Buckingham Palace, Downing Street, etc. But they never managed anything like it.
The Muslim terrorists are much better funded than the IRA ever was, and they still haven't done this - yet there is zero doubt they would do this in an instant. Not a sovereign state, overtly (that would just cause some ICBM launches, wiping it off the map an hour later) but terrorists.
I think that building a bomb is simply much harder than people think. The simple designs are big and heavy (tons) and one would need something bigger than a bizjet to carry it. And it needs a lot of material which - in that volume - doesn't fall off the back of a lorry - not even at BNFL. The sophisticated compact designs are ... sophisticated and difficult to build, and those who have the designs have exactly zero interest in leaking them because the result could be used against them next. And you certainly cannot find the designs for the latter type on google. Not working designs.
If it was that easy, the things would be going off all over the place - delivered by vehicles.