Most people use a 200 watt infrared heat lamp. They are mounted in a 9" or 10 " or so aluminum reflector with a clamp on it to hang it. Do a google search for "brooder heat lamp" to find a picture. Most feed stores have them, I've also see them at Lowe's and Home Depot usually sold in the electrical section as a shop light. Make sure whatever fixture you use is rated for the 200 watt lamp.
You want to hang it so that it just shines in part of the brooder, usually the center, so they can get away from the heat by moving to the outside edges of the brooder. That way the chicks can move closer or further from the heat as needed to adjust. If they are all hanging out directly underneath the lamp they are too cold and the lamp needs to be lowered a little to provide more heat. If they are all avoiding the area underneath the lamp and are packed up against the edges of the brooder then it is too hot and the lamp needs to be raised a little.