I have a YT3600 and I just went through the search to find less expensive belts.
I found the 'LB' numbers (just like your LB67) listed in the back of the owners manual. I searched Google using just the LB number and got several hits. (I checked one of the part numbers you supplied and it didn't come back with anything.) In one case, I was able to find a John Deere equivalent on ebay and I was able to pay less than $20 including shipping for a belt that Yamaha would have charged $50.
If you don't have the owners manual, either someone on this forum might be able to help find the LB numbers, or I would recommend tracking down a copy of the manual.
Well I think he is right about the LB numbers being the length but not about automotive belts being sustitued. I just got a 6800 from a girlfriend whose dad died 13 yrs ago. I have no manual and was missing a PTO belt and went to the local automotive dealer and screwed around with all kinds of sized belts. The PTO belt is probably 67 inches long but as soon as you install them they streach out of shape. I am now trying to find a proper belt up here in Vancouver BC. Any sugestions?