Impregum works. If you really want something that works you can by fixing gel online at the Columbia typodont website, which is what is required to mount teeth for the endo portion of the clinical licensing exams most places. The only catch is, it is very difficult to get the fixing gel out of the typodont vs. Impregum or PVS. If it is just for practicing, the Impregum or PVS will work, just make sure you put some notches in the teeth so the impression material will "lock in" to the tooth.
