Where can I find small2e.tex and sample2e.tex in the MacTex installation?

They should be in the core latex distribution so you should be able to go from anywhere

pdflatex sample2e

and the file should latex then sample2e.log should show the directory where it is located.

You should have a command line tool called kpsewhere that searches the texmf tree. Therefore by

kpsewhere sample2e.tex

it should output the path to sample2e.tex.