Do you mean like a 'drag dynamo;' one that attaches behind the BB shell, with a roller that drags against the tire? Or one that's wholly contained inside the BB shell & generates power directly from pedaling?

If the former, I'm pretty sure Sanyo still makes one; and I'd swear I recently saw one made by Axa, but I'm not totally sure about that (it's been a few years & my memory is a tad fuzzy; I might be thinking of an NOS vintage unit).

I was exploring these for a while myself as one of my 650b bikes (my Panasonic) has a brazed-in bracket for mounting a BB dynamo.

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