Here you go Johnny.. good luck attaining enough weapons grade plutonium or uranium to make it feasible!
Plutonium239 isotope. Around 25 pounds (10 kg) would be enough. If you could find some Uranium235, that would be good, but not great. You would need to refine it using a gas centrifuge. The uranium hexafluoride gas is piped in a cylinder, which is then spun at high speed. The rotation causes a centrifugal force that leaves the heavier U-238 isotopes at the outside of the cylinder, while the lighter U-235 isotopes are left at the center. The process is repeated many times over through a cascade of centrifuges to create uranium of the desired level of enrichment. To be used as the fissile core of a nuclear weapon, the uranium has to be enriched to more than 90 per cent and be produced in large quantities.
You could try buying it from a former Soviet Republic, or from Iran, since they're trying so hard to produce it. North Korea is not ready yet, and unfortunately, Iraqi dealers retired from the business.
b. The explosive to start the nuclear chain reaction
100 pounds (44 kg) of trinitrotoluene (TNT). Gelignite (an explosive material consisting of collodion-cotton (a type of nitrocellulose or gun cotton) dissolved in nitroglycerine and mixed with wood pulp and sodium or potassium nitrate) would be better. Semtex would be good too, but it's a bit hard to get, these days.
c. The detonator
To fabricate a detonator for the device, get a radio controlled (RC) servo mechanism, as found in RC model airplanes and cars. With a modicum of effort, a remote plunger can be made that will strike a detonator cap to effect a small explosion. These detonation caps can be found in the electrical supply section of your local supermarket. If you're an electronics wiz, you should be able to make it using a cellphone.
