Uxcell has quite a good range of mp3/mp4 players at low prices, they also have some spares like screen glass and spare batteries:-
http://www.uxcell.com/mp4-mp3-access...-981_1613.html
Tinydeal also have a good selection:-
http://www.tinydeal.com/
If you are looking for something a bit better, but still at good low prices then Mp4nation are probably the best:-
http://www.mp4nation.net/catalog/
Hope this helps..