What are some dense elements I can use for a demonstration? I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a $10 {\times} 10 {\times} 10 \, \mathrm{cm}^3$ cube of iron, lead etc. So far, the densest material accessible and affordable to a teacher would probably be mercury (I certainly remember my chemistry teacher letting us lift a small bottle, maybe $100 \, \mathrm{mL}$, makes for an unforgettable impression).
Do I have any chance of getting anywhere above the $13 \frac{\mathrm{g}}{\mathrm{cm}^3}$ of mercury? I sorted the elements in the Wikipedia Density article. Gold and Platinum require to win the lottery first. A liter of Plutonium needs connections to evil people, plus a lot of safety measures, so is right out :-) Osmium? Rhenium?
Are there elements denser than mercury usable for a demonstration?
 A: There is a rather lovely looking set of spheres available now through an indiegogo campaign - these are 1 kg objects of different densities. It is a more visual way of driving home the message that you wanted to show with your demonstration:

At $890 for the set it is beautiful, but rather expensive for a science demonstration - more something for a CEO's corner office.
However, there is a set of "density cubes" you can buy on Amazon - probably more in line with what you are looking for. It is sold by Midwest Tungsten Service (in case the Amazon link rots).

Here, the volume is constant but the different densities make it interesting. Note - the title says "Tungsten carbide" (density $\rm{15.6~ g/cm^3}$) but the engraving suggests "tungsten" (density $19.3 ~\rm{g/cm^3}$). At $130 for the set it is less out-of-reach.
DISCLAIMER: I have no relationship to either of these vendors and am providing the links purely as a convenience to people visiting this site. These links may not last...
A: Here is a table I made for you listing the elements with a density higher than $10 \frac{\mathrm{g}}{\mathrm{cm}^3}$ and their approximate price per kg:

I couldn't find any prices for Einsteinium or Actinium and some of the other prices might come from poor sources, but take it as a rough guide.
Now you only have to figure out how much you need and your budgetetary constraints, and choose the densest you can afford. As I have learned from the political debate in the US, teachers are apparently raking in big cash, so I suggest you go with osmium or rhenium.
Note: Some of these might be unsuitable/infeasible for other reasons than their price.
A: I'm not sure if you want to stray from the 1 liter amount or not but searching via Amazon it seems there is a company selling spheres of quite a few different metals.  They have tungsten, steel, brass, chromium, etc.
See here and here
