What alternative to slavery do you think the colonists could have used to solve the labor shortage

For one, there wasn't exactly a labor shortage. Because of the economic theory of mercantilism, the colonies were established to produce a surplus of goods. In other words, the only reason there was a labor shortage was because the colonies had been established in the first place for the purpose of creating tons and tons of mostly unnecessary stuff (like tobacco). The best solution would have been simply not to colonize America in the first place, but the colonists could also simply have done the work themselves, or hired labor from other parts of the world rather than capturing and enslaving people.