Asking for sources is "sealion". Well maybe find a wikipedia article that isn't controversial, or you know maybe look at the sources that are cited there.... Jstor allows you to access 100 free articles a month... So there are academic sources that are freely available.

So you posted something on Nangzen... but you clearly didn't read it (despite it being only one sentence long...) "According to Chinese government sources, Nangzan (also nangzen, nangzan, nangsen) were hereditary household servants comprising 5% of the population."

So the best thing you can post, is a one sentence wikipedia link where is starts off with, "According to the Chinese government..." Do you really not see how this is an issue?

Oh here is a source on Nang Zan.

http://himalaya.socanth.cam.ac.uk/collections/journals/ret/pdf/ret_37_16.pdf

So again, do you have a source for slavery in Tibet?