A few examples:

Amazon.com : Planet Bike BRT Strap Multi-Use LED Bicycle Safety Light : Bike Taillights : Sports & Outdoors

AGPtek 5 Packs LED Flashing Snap Arm Band Flashlight Shiny Armband for Running Cycling Cheering Squads Festivals Concerts Promotions Events Dogs Walking Safety Reflective Belt Strap - - Amazon.com

Amazon.com : Safety Reflective Flashing LED Glow Armband Visible Belt Sports Hiking Jogging : Sports & Outdoors

From the reviews the first one (Planet Bike) sounds like of good quality. I got the cheap one in the one in the last link. They use button batteries, probably not as bright as the Planet Bike product but better than simply reflective straps that I used to use. What especially like is they don't use velcro. They are nice even just for the elastic to hold one's pants in windy weather.

Thanks for your reply. I haven't seen such products at a few of the (quality) bike shops I visit but maybe I didn't look too hard. The new lights are Planet Bike "Spok" [sic] though not spoke lights but can go onto the wheel hub according to package.

PS: I don't recall seeing lit leg bands on other cyclists out on the road either. I cycle-commute in the early morning in the vicinity of Kenmore Square, a major transportation hub, and the Harvard Medical School and Hospitals, partly on a MUP, so I see a lot of well-lit cycle commuters. I do recall once seeing a well-appointed rider with right and left reflective leg bands, and that caught my eye. I started to wear both leg bands soon after that encounter.

I have seen a few runners with illuminated arm bands.