what lightbulb can I use in this 250V 100W light fixture?
The numbers you have found are the maximum ratings of the socket.

The wattage of the bulb, if incandescent, must not exceed 100 W but may be less.

The voltage of the bulb must match the voltage supplied by the electric service, which is almost certainly whichever of either 120 or 240 volts is standard for your country. The voltage rating of the socket is irrelevant. Check the old bulb for a voltage marking.