As everyone is saying, there are tons of resources online to look for your parts and also many manufacturers. I'm attaching an illustrated parts diagram of a King Kutter 40hp gearbox.
If you can identify your cutter, I'm sure we can help. If you can't identify it or get a picture, you could search for months before you find out what brand it is.
If you are interested, I do know from first hand experience that many gearboxes from different manufacturers are interchangeable. It just depends on the design of your cutters blades and stump jumper (if it has one) and the spline type. It's also critical to know if the gearbox is 40hp, 60hp, 80hp, etc..