In biosynthesis of cysteine and methionine, where does the sulfur come from?
Microorganisms (and plants) make cysteine from serine using H2S as the sulphur source. Condensation of these two directly gives cysteine in some bacteria, but most often the reaction sequence is:

serine + acetyl-CoA &gt;&gt;&gt; O-acetylserine + CoA-SH
O-acetylserine + H2S &gt;&gt;&gt; cysteine + acetate + H2O

As you say, in terms of the sulphur, cysteine and methionine are interconvertible.
Animals rely upon their diet for methionine, and can make cysteine from that methionine.