A brushless DC motor can be used as a 3-phase alternator of you're not fussed about squeezing the 99th percent of efficiency out of it. Just buy any cheap brushless motor from banggood or hobbyking and six diodes. Connect as below:
If you want refinement add a smoothing capacitor.
You can make educated choices about the motor that best suits your needs if you know what voltage & current you're after and what operating RPM you're looking for. The main thing is to know what current you'd expect your load to draw - ensure the motor and diodes are rated for that current. The motor constant (Kv, expressed in RPM per volt) gives you a reasonable idea of how fast you will need to turn it to get any given voltage.
AS