I hate to say this, but based on available information at Deere.com and using their configurator for a 5105, I do not think they offer a creeper kit on the 5x05 tractors.
It would appear that the standard and high speed creeper kits are only available on the 5x20 and 5x25 series tractors. And the only place they are even mentioned is in the configurator. There is no mention of creeper kits in the normal specification and information pages for the 5x20 or 5x25 tractors. I also don't recal them being mentioned in the 5x20 or 5x25 brochures either.
Of course, the best place to get definitive info on possible creeper kits for the 5x05 series tractors is going to be your local Deere dealer.