That's one of those things that annoying. It's cosmetic, but it really can make the difference between a good looking tractor and a bad one. Not to mention, it can let water in there and foul up the sockets for the bulbs.
Found it on Amazon, Golden Eagle, Coleman Equipment, and Tractor Help. Amazon being the most expensive and Tractor help being the least expensive. Figuring you're "thrifty" like most of us on here, here's the link to the cheaper one. LOL Replacement parts for the Kubota L2800DT Tractor