Voltage stabilization
Based on the question you said the motor requires 5v thus you need a source that can provide 5v with a lot of current, I'd recommend you get a 6v SLA battery, it may be 6v but it's still safe to be used on 5v applications yet at the same time SLA batteries have a very low internal resistance thus there barely will be a voltage drop when you connect the motors and the SLA battery provides a lot of current. So try a 6v SLA battery, with a diode in series to keep it within the 5v requirement