You can get cubes of most of the solids in the periodic table here.[1] Some are very expensive, but available.
It seems to me that the point of buying a cube of some metal is to feel its weight and texture, to see it respond to light. If you're just going to keep it in a capsule and never take it out, you could just have a photo or video of the metal.
What elements can you hold in your hand without either you or the element taking damage?
Some of them will oxidize and if you want to stop that you can coat them in a clear resin. I wouldn't touch lead even with the resin coating as I wouldn't trust it to not chip off.