FG-280 replacement machine tuners?

It looks to use the common tuner of that era, shared with the FG-160, 170 & 200. You could find another guitar for parts.

But, I assume you want to replace them with something new. I don't have one of those models to look at right now (I'm at work), but typically the vintage Yamaha tuners have 6mm (.236") shafts vs. modern 1/4" (.250"). Anything you buy would require carefully pressing the tuner bushings out without damaging the headplate. Not sure if the holes in the headstock will have to be opened.

Do you still have the original tuners? if so, you could measure them and compare them to online drawings of modern tuners. Stew Mac, LMII and All Parts are common sources. I've had good luck buying from Arkansas Music Works store on eBay. Discounted prices and free shipping. LMII also has free shipping.