Depends on your budget I guess.
But id suggest one of those low output permanent magnet alternators of small engines such as Kubota as a starting point if you want it to last, but wont be cheap, and will need a reasonable amount of input power to turn.
A cheaper alternative would be a windscreen wiper motor off a late 80's commodore with a ball bearing on the output end of the armature. Cut away the reduction gear housing and drive the armature directly. Will have to keep the bush end of the shaft well lubricated.
Any small permanent magnet motor will work as a generator, but if you want a storage battery then you need to think about regulation and reverse current.
The higher the voltage ie 24 volt the motor is the sooner it will produce power at lower revs but not produce as much current, if you really get into it and the budget allows, get the armature rewound so suit your parameters.