WordNet records no synonym and no antonym for allomorph. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
any of several different crystalline forms of the same chemical compound
“calcium carbonate occurs in the allomorphs calcite and aragonite”
a variant phonological representation of a morpheme
“the final sounds of `bets' and `beds' and `horses' and `oxen' are allomorphs of the English plural morpheme”
Synonyms, antonyms and the broader and narrower relations for allomorph come from Princeton University’s WordNet. Full sources and licences.