A velocity source is such that it provides a velocity independent of force . Of course these are usually idealizations - it's the rare bulldozer so strong it can totally ignore a force exerted on it.
Idealization is essential to modeling.
Also, keep in mind that a source does not need to provide a constant force or velocity. The point isn't that it be constant, but that it be independent . For instance, a bulldozer can provide a time-varying velocity because the driver is switching it between forward and reverse. The velocity can depend on time and we'll still call it a velocity source, but if it depends much on force we won't be able to model it well as a velocity source. Finally, as in the example of the tugboat, how we choose to model a device may depend on the context.