Most of the chain autoparts places have 12v led kits in various colors for this and other purposes. Many have a decent length pigtail. Find a dash light wire to tap and a ground stud. I would drill a hole in the back of each cup holder and fish the wires through the hole, under the carpet to the dash area where you found the dash light wire. Drill the hole so that the fit is snug and so that the led doesn't stick out too far. You may need to color the top with a black sharpie or similar to prevent glare. Depends on the led.

A variation on this theme is to install one that shines down on the cup holders from the console area.

We do this for some control components don't have panel lights.