Can you buy a gearbox?

I'm curious, can you actually purchase a gearbox? I understand that gearboxes are intricate mechanical components used in various applications, from automobiles to industrial machinery. But are they readily available for purchase, and if so, what factors should one consider when making such a purchase? For instance, do you need to have a specific make and model in mind, or are there general-purpose gearboxes that can fit a variety of needs? I'm eager to learn more about the process of acquiring a gearbox and any potential challenges that may arise during the buying process.