You can buy mp3 players that have their own remotes. The cord to connect them would have male plugs on each end and be specified by the stereo line out jack (assuming it has one) and the input into the PA system (typically 1/4" or 1/8" stereo).
Another option for playing the mp3 is to burn it to a CD as an audio disc, and you can use any CD player with a remote or footpedal.
Steve Jones makes both an Mp3 and CD player with a foot pedal to start and pause audio. Here's his link .