Is not having/finding tenants a real concern? what do you do if you can't find any?

[Sarah Ali]

One of my concerns with buying a rental property is not finding tenants (or worse, finding bad tenants). Is this a real concern? How do you mitigate it?
Where do you get the money to pay for the mortgage for those months it sits empty? (Obviously this question is for if your monthly incoming from other properties and/or W2 don't cover it).