You can find small DC generators for bicycles readily in the marketplace. Bicycle Generator search
For making it into 5V, you'd need a simple 5V linear regulator like an LM7805, or LM78M05... there are many variations, depending on power requirements. Google - 7805 search
Be aware that a 5V regulator will only step a higher voltage down to 5V... it won't increase 3V to 5V. 3V in will be 3V out, but more generators will give you something higher than 5V, IIRC.