If the roller isn't broken or flat on one side from wear due to insufficient lubrication, don't bother replacing it. Most window rollers do not actually roll, they slide. Thoroughly lubricate the tracks in which the rollers "slide" and it will function as if it were new. Use a medium density lithium grease, such as white Lubriplate (GM #1052196) or similar.
Watch out for razor sharp edges and don't operate the power window while your hands are inside the door!