Kostoulas stayed for a year at his first professional club, Olympiacos Volos, before moving to Kalamata for the in 1994.
He started the following season on the bench but when handed an opportunity he took it, playing in seven out of the final eight games and excelling in the derby defeat of Panathinaikos which paved the way for another championship success.
He remained a consistent performer when used as the title was lost in 2004 and then won back in each of the next two seasons along with the Greek Cup.
Kostoulas was a member of the Greece side reached the 1998 UEFA European Under-21 Championship final, losing 1–0 to Spain.
He was then elevated into the senior fold, making his debut in a 4–2 FIFA World Cup qualifying defeat by Germany on 28 March 2001 but has not figured again since a second cap a month later against Croatia.