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Most easiest place to enter Tibet is through Chengdu as there is lots flights to Lhasa and also it will be almost the most closest mainland city to Tibet. You can easily buy flight ticket to China through any website which is cheaper and then travel into some of the beautiful sightseeing in Chengdu by your self as all the palces in Mainland China you dont need any permit or Tour Guide only the China visa is o.k. There is a big Buddha Stone statue near Chengdu. Le shan da fo. then you can take plane to Tibet. If you want to take train to Tibet then Xining will be best option as its only about 25hours in train.