obtaining a Visa abroad

Taylor, it is mainly up to from which country you will apply for it because in some countries, the residence permit on that country is needed. But for some countries, you can just the valid entry permit and other relevant documents to apply for it.
After you get an entry permit in a third country and enter PRC, you need to find a travel agency to get the Tibet Travel Permit for you and then book a tour to enter Tibet.
Please note that when you apply for the entry permit to PRC, you’d better not mention that you want to visit Tibet.