They are simply called geared motors [used in robotics]. They are either 6V or 12V rated. Buy the one with 12V rating.
You have to buy a pulley which firmly fits on the shaft where you can attach the handle as I have shown in the picture.
See the catalogue for small geared motors on any shop or website. You better go for gear motor with metal gears for durability and buy the one which you can hold firmly in your one hand. With all the geared motors they will specify the gear ratio. 10:1 means when the motor rotates 10 times the outer shaft will rotate only 1 time. This way you have higher torque output.
Vice versa when you operate motor as a generator the it will become 1:10 gear ratio which means when you rotate outer shaft 1 time, the motor rotor will rotate 10 times.
For example see these
https://www.pololu.com/category/51/pololu-metal-gearmotors
Motors have gear ratio like 10:1, 20:1 etc
Now connect a DC-DC converter[based on LM2576, See the datasheet for ckt diag.] to hand cranked generator's output to get a steady 5V output from the DC-DC converter.