Harvey,

Today it is in Ukraine.

https://www.jewishgen.org/Communities/community.php?usbgn=-1043071

Kostopil' [Ukr], Kostopol [Pol], Kostopol' [Rus], Kostopel

Region: Volhynia

You can search at:
https://www.jewishgen.org/databases/Ukraine/
for records from the town.

Good luck in your search,

Sherri Bobish