The phrase "are extant in" is correct and can be used in written English.
It means that something still exists or is currently present in a particular place or setting. Example: Many artifacts from ancient civilizations are extant in museums around the world.
The Ruckers shop built variations of nine basic models of harpsichords and virginals, of which more than 100 are extant in whole or part.
They are extant in Ethiopic, Syriac, Greek, and Latin translations made by Christians rather than in their original Hebrew or Aramaic forms.
Ulfilas translated the Bible into Gothic; of this translation, some fragments are extant in manuscripts of the 5th and 6th centuries.
Puebla has been repeatedly damaged by strong earthquakes, but numerous colonial-era buildings are extant in the grid-patterned city centre, which was designated a UNESCO World Heritage site in 1987.
They are extant in the Arabic original or Hebrew translations or both, and some of these translations serve in place of the presumably lost Arabic originals; e.g., the important commentaries on Aristotle's Nicomachean Ethics and on Plato's Republic.
A variety of other forms are extant in the literature (e.g., Refs. [10, 14]).
And: I would like to be extant in another version.
The work is extant in several Greek manuscripts and in Armenian and Slavic translations.
The artist has erased the rest of the word, which is extant in the title.
It is extant in a 2nd-century Latin translation, which is possibly the oldest surviving Latin Christian work.
A pious romance of Christian apologetics, the legend is extant in several versions, including Greek, Syriac, Coptic, and Georgian.
