Let's look at the simplest atom, hydrogen. The majority of hydrogen atoms are the #""^1H# isotope; their nuclei contain the 1 proton, the one positively charged nuclear particle. A few hydrogen nuclei contain the 1 proton (this is of course what defines them as hydrogen atoms), but also 1 neutron in addition, to give the deuterium, #""^2H#, isotope; a smaller fraction of hydrogen nuclei contain 2 neutrons, to give the tritium isotope, #""^3H#.
I stress that all of these nuclides are hydrogen atoms, because #Z=1#, but they are isotopically different because of the presence of small fractions of the heavier isotopes, i.e. their mass is double or treble because of the presence of the heavier nuclide. As #Z# increases, the nucleus can support different numbers of neutrons, and thus for heavier elements there is an isotopic distribution.
The mass quoted on the Periodic Table is the weighted average of the individual isotopes.
The element tin, #Z=50#, has 10 stable isotopes, and the atomic mass quoted on the Periodic Table, #118.710*g*mol^-1#, is the weighted average of the individual isotopes.
As isotopically labelled materials go, #D_2O(l)#, or #D_2(g)#, i.e. the so-called isotopomers of water and dihydrogen, are RELATIVELY cheap. That is, deuterium or heavy water are expensive, but because when these are incorporated into your reference compound, your compound becomes ISOTOPICALLY distinct, and thus MAGNETICALLY DISTINCT, i.e. #""^2H# #NMR# spectroscopy becomes possible, which is nowadays a routine experiment, and its results may be powerful and discriminating.