He probably already does have one, and it would be surprising if nobody in the audience does not have at least a good cellphone camera.
Yes, if Trump were smart, he’d tweet out a request to anyone who may have been sitting in those seats before leaving them empty to go up to the podium.
As you imply, at least some of them may have been taking pictures with their mobile phones. They could forward the pics to him or post them to instagram or reply to his tweet with a twitpic of the filled seats.