Where does “-а” come from?

-а (Ukrainian) comes from Proto-Slavic -ja, from Proto-Indo-European -yeh₂.

Ancestry of “-а”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ja	Suffix creating feminine action nouns, usually...
2	Proto-Indo-European	-yeh₂	—

-а — Etymology, Origin & Word History | Etymology Explorer