I Googled stirrup lights but apparently they don’t make them anymore; I remember reading about someone having one in one of the old English horse novels.
Good tips from the BHS:
Rule 50: Other clothing
You should wear boots or shoes with hard soles and heels, light-coloured or fluorescent clothing in daylight, and reflective clothing if you have to ride at night or in poor visibility.
Rule 51: At night
It is safer not to ride on the road at night or in poor visibility, but if you do, make sure you wear reflective clothing and your horse has reflective bands above the fetlock joints. A light, which shows white to the front and red to the rear, should be fitted with a band to the rider’s right arm and/or leg/riding boot. If you are leading a horse at night, carry a light in your right hand, showing white to the front and red to the rear, and wear reflective clothing on both you and your horse. It is strongly recommended that your horse also wears a fluorescent/reflective tail guard.
I guess here in the US that would be the left arm and/or leg/boot.