where are you at?

is "Where's my wallet at?" a correct sentence?

No, it's slang.

No, the ''at'' is unnecessary. ''Where's my wallet'' works just fine.

Let me elaborate. "At" is a preposition, and it is incorrect to end a sentence with a preposition.

If you're trying to speak slang, the "correct" way to say it is "Whe' you at, dawg?" ;-)

What about "Who is he with"?