Nature's alphabet ends with the letter U. The Strong Nuclear Force, the far less well known sister to gravity and electromagnetism, is responsible for sheepherding protons and neutrons into tidy family units. But the SNF's mojo can only carry things so far. Once a family grows too big things can get ugly and relations can break down. Explosively so. There is no magic number but in the case of 235, well, that happens to be an especially troublesome number of family members corraled into one condo which we'll call an "atom". You see, when a dispute breaks out in Uranium clan 235 the violence can break out into neighboring atoms and all hell breaks loose.
Oddly, if a couple more cousins show up things aren't so bad. Uranium-238, otherwise known as depleted uranium or "DU", is of the biggest "pretty stable" atoms that strong nuclear force can manage. That means the metal is actually quite low in radioactivity, relatively speaking, and the type of radioactivity it emits is composed of a comparatively heavy 4-man team composed of two protons and two neutrons. In essence, when uranium breaks apart it farts high-speed helium. And that just doesn't hurt much.
A little more seriously, that's not to say DU is harmless. If allowed to turn into airborne dust it will lodge in your lungs and pepper your alveoli like a machine gun with infinite bullets. While they may not hurt much at any one moment the cumulative effect over years can wreck DNA and degrade tissues into cancerous tumors. Not good.
We offer small chips of DU for sale stored in oil and shipped in vials (as well as embedded in cubes). The U.S. Nuclear Regulatory Commission allows for the sale and transportation of uranium to the public if certain conditions are met; the most pressing of which deals with the maximum allowable quantity any one person may own (1.5kg) and that it may not be exported outside of the country. For full details please see NRC §40.22 If you can live with these limitations then you should definitely consider owning a little of this marvel of nature.
As stated above, we are unable to ship uranium internationally (sorry).