If a thing is very common and easy to get, we say it is "a dime a dozen." Example: "Do you think I should buy this now and bring it with us?" Answer: "Don't bother; those are a dime a dozen where we are going." There is no need to get excited or worried about finding something that is a dime a dozen. Example: "Look what I found!" Answer: "That's nothing special; those are a dime a dozen." It is easy to find a dime (a 10 cent US coin), and a dozen (12) of something is a common, everyday unit of measure. You are not in a hurry to get a thing which is a dime a dozen because it is not so special and you could get one any time you wanted. Example: "I don't need friends like him; they are a dime a dozen."