We are travelling in June and will to check what is the prepaid data plan that is available in Tibet

If I understand correctly, you would like to prebook a tour there, don't you? If so, the TBT Entry Permit shall be needed and it can only be applied through a travel agency. Then I recommend you use the service of Travel China Guide, who is good at tailor-made itineraries. If needed, here is their email: tourstravelchinaguide.com