From the owner's manual--
Blade belt is LB103 (6800 only)
PTO belt is LB67 (5700,6700,6800)
Drive belt is LA86 (5700,6700,6800)
I am not certain but the numbers may be the length of the belt (in inches?). I have not confirmed this though. An alternative is to take the belt to your nearest Auto Parts store and see if they can match them up. I had to do this for a JD318 I have when my belt broke in the middle of mowing.
OEM Belts can be gotten from...
http://www.yamahagenuineparts.comhttp://www.canns-bilco.comhttp://www.powersportspro.com/pages/parts/viewbybrand/2/Yamaha.aspx
Rattosh51
YT6800