Unfortunately, as parts for older bikes get used up, prices increase. I've had good luck with this outfit for rare parts.

3UH-F582U-00-00 - Disc, Brake (Right), Yamaha OEM | Megazip.net

3UH-F582U-00-00 - Disc, Brake (Right), Yamaha. New spare parts Yamaha from Japan with easy payment system and door-to-door delivery.

www.megazip.net

 

Thanks- that site is a great resource- bookmarked!! I just found a salvage real OEM one on ebay that had clearly been sitting a lot- a bunch of rust- but nothing actual brake use won't get rid of, and it's still basically at 4mm- for $40.. But yeah- I feel like I I could keep this scooter running almost forever as long as I can keep getting reasonably priced parts..

I messaged the seller of the aftermarket "new" one with the 3mm stamp, and they said 3mm was the min service limit, and the rotor itself is 3.8mm- both numbers below official Yamaha specs .. Salvage or NOS from sources like that ^^ from here on out I guess for the parts that are no longer sold.. Thankfully a lot of basic ones still are!

 

You can remove the rust with steel wool followed by a cleaning with brake cleaner

 

In addition to thickness numbers, you'll want to gather info on materials used. Factory part tend to be cast-iron or steel. Aftermarket part may be 420 stainless and much stronger. If so, it can be quite usable down to 3mm and last twice as long before getting there.

I replaced factory discs on my Ninja and CBR racebikes after they wore out in 2-3 yrs with Galfer 420 versions. They're both over 10-yrs now and on their 3-4th set of pads. Holds up much better against more aggressive sintered semi-metallic pads compared to factory discs.