Can you buy liquid nitrogen and mercury? If so where?
While it's not too difficult to find suppliers, since they are regarded as hazardous materials, on any (legitimate) site that you find that sells them, you're going to find a caveat like this:
"Hazardous chemicals will be shipped only to schools, universities, laboratories, and businesses that can provide proof of use and carry liability insurance" (sciencestuff.com)
I'm going to guess you aren't writing from a laboratory...
Liquid nitrogen should be easy enough to buy from a chemical supplies company - its only danger is that it's really cold. Mercury, however, will be really hard to obtain in appreciable quantities, as it's quite poisonous. You'd better have a legitimate reason, and some certification to say so.
you can im not sure where thought, some science place perhaps?