If you'd like to buy any of the Stable Elements, Alfa Aesar sells most of them.
Here's a problem that really bugs me: the Stability of the Atom. I made a chart of the stable and semistable isotopes. The table is arranged by the excess number of neutrons.
Could quarks explain the above table? You can read up on quarks at the Particle Data Group page. The page is aimed at non-physicists, and is delightful. For physicists, you might enjoy The Weak Interaction in Nuclear, Particle and Astrophysics by K. Grotz and HV Klapdar -- it's a very hard book. Let's suppose quarks exist, and that a simple explanation exists for atom stabililty. Maybe, each Proton (uud) and Neutron (udd) can be considered as a triangle. The problem would become geometric. Why is it that Stable Isotope A frequently can be made into Stable Isotope B by adding two protons and two neutrons? Perhaps because Helium-4 (2p 2n) is a tetrahedron, and adding a tetrahedron often doesn't affect stability.