Is it possible to get a visa for Tibet in Kathmandu?

Hello, I have Argentinian passport and I would like to apply for a Chinese visa at the embassy in Kathmandu as I am planning to visit China and Tibet. I know a second visa is necessary to enter Tibet and I red in a comment above that it would be necessary to apply for it through a Chinese travel agency. Is it possible to do that here in Kathmandu or need to be in China for that? Once I have the visa, is it possible to travel on my own through Tibet? Or I would need to buy packaged tours at the Chinese travel agency?

Thank you so much for the information.

Best regards.