There is another active thread where the poster was asking the same question, as was pointed to some MP3 shields that could do what you want: Arduino mp3 playback with remote - Project Guidance - Arduino Forum
Here is another one from a US supplier I've bought other components from: http://www.ebay.com/itm/MP3-Music-VS1053-shield-board-with-TF-card-slot-work-with-arduino-/321157017822?pt=LH_DefaultDomain_0&hash=item4ac67330de
A somewhat cheaper approach would be to use a WTV020SD16P based card that plays WAV files (note, many of these WTV020SD16 devices will only take older, smaller SD cards up to 1GB): http://www.ebay.com/itm/WTV020-SD-16P-U-Disk-Audio-Player-SD-Card-Voice-Module-MP3-Voice-Module-/221263135372?pt=LH_DefaultDomain_0&hash=item33844f828c