Is there a contraction for "where are" like "where's" for "where is"?

What is a contraction for where are?

I wanted to ask "Where are the keys?" but want to use a contraction.

Do I have to just say Where are, or can I say Where's and it's understood?

"Where're the keys?" = "Where are the keys?"

"Where's the key?" = "Where is the key?"

Your question revolves around the number of the word key.

You will commonly hear utterances such as Where's the keys or There's cookies in the jar or Here's the answers to the quiz. David Crystal has a useful article on this topic. He states that where are and where're are "much more awkward to pronounce".

He continues:

So what is the grammatical rule involved in the informal cases? Basically, the verb is agreeing with the meaning of the following noun phrase and not with its form. The distinction is summarised by calling the former principle notional concord and the latter principle grammatical concord. Traditional grammars always insist on people following grammatical concord, but spontaneous speech contains many examples of notional concord taking place.

As to the OP's last question, it is difficult to imagine a context in which you would be misunderstood for asking Where's the keys? etc. rather than Where are the keys? - unless you misheard keys as key.

David Crystal: Safety in numbers (.pdf)