After a second thought about your question regarding direct-acting mutagens
to use in an Ames test demonstration--
The agent that you should choose depends on the nature of the mutation
needed to revert the bacterial strain(s) that you are testing. UV light will
primarily affect sequences with Py-Py steps, so it may not be particularly
effective for a strain that requires a frameshift or some other mutation for
reversion. So your question of which agent to pick is compounded by the type
of mutation needed to see a good reversion/mutation frequency.
I have used UV light from an apparatus used to crosslink nucleic acids to
membranes for southern and northern blotting as a mutagen. This apparatus
measures the energy delivered, so you can calculate the UVL dose for
Sodium chromate and dichromate are cheap and effective mutagens that don't
require a microsomal activation system to work, and you may have them around
the lab already.
1 gram (probably a lifetime supply) of methanesulfonic acid ethyl ester
(ethyl methane sulfonate, or EMS) is ~ $14 from Sigma Chemical (product M
0880, www.sigma.sial.com, I have no interests or affiliation with Sigma). I
would think twice about turning students loose with EMS, and you will need a
hood and other safety measures.