Make one. Strap or weld a tube on the loader cylinder mount where you can see it. The run a rod thru it down to the bucket cylinder mount and pin it there. (Not the bucket because if you swap it out....) Set the bucket to a zero position of your choice and put a piece of electrical tape on the rod as it enters the tube. After a while you'll get used to it and recognize the level position just from the cylinder length.
Don't laugh, but I've also seen someone putting a camera up there. GPS is not accurate enough ( Theoretically), but I'm sure there's a propeller head out there willing to try it.).