I wouldn't worry about finding an auger that is ready-to-mount to an L555. It is worth a try, but not essential since the mount is just pinned to the drive head.
First, find a used earth auger...preferably for a universal, Bobcat-type mount. Bobcat-type mounts are readily available, since it is the most common mount.
Once you find the used auger, have your dealer order a mounting plate for the L555 New Holland. McMillen or Lowe will have those plates readily available for their auger drives. The price for the L555 mounting plate will be reasonable. Once you have the mounting plate, it is a simple 5-minute job to switch to the new mount.
Since you will now have two mounts, the auger could be used for a newer, universal Bobcat-type quick attach if you ever trade skid steer loaders.