Open it up, take the brushes out and see what they are like. Give the motor a clean with some fine sand paper or wire wool (just lightly to remove any carbon or grease build-up on the commutator where the brushes contact the motor). If the brushes need replacing, you might be able to identify similar brushes (for example a newer model of the same brand of drill) or carve your own from a suitable block of carbon (such as the core of a large dry-cell battery).
All excellent advice, if your are dealing with a valuable tool, or your time is of no value. Take the thing back to the thrift store and trade it in for another one. The other lesson here is to never buy a craftsman power tool. Sears went out of their way to deviate from standard designs, and parts, to make their power tool users sickly dependent on the company for parts and repairs. They made good hand tools and I have many, however, I wouldn't spend $5 for a craftsman drill at a yard sale, much less buy new one at their retail stores. It doesn't matter what you need to keep the tool in service, a chuck, trigger, or brushes, if it's a craftsman product, it can only be repaired with obscure, expensive, or is your case, unavailable, sears sourced parts.
IMHO, when it comes to commonly available consumer grade power tools, new or used, the top shelf is Bosch, Makita, Milwaulkie, Dewalt, and Hitachi. The next tier would be Ryobi and Ridged, and "only if it's cheap enough to throw out when it breaks" category is anything found in a Sears or Harbor Freight store.