There are two kinds of permits (Tibet visa) for traveling in Tibet: the entry permit (TTB permit) and the Alien's travel permit (PSB permit). You can get them easily from Access Tibet Tour.

TTB Permit - Tibet Entry Permit (Tibet visa)
When you fly from any city to Lhasa you will need to show this permit when you check in at the airport. Furthermore, whenever you buy a flight to Lhasa you need to show this permit. TTB permits are also needed by groups traveling by Land Cruiser but this will be arranged by the travel agency organizing the trip.

PSB Permit - Alien's Travel Permit
If you want to go further to the "unopened" areas, you need an Alien's travel permit issued by PSB of Tibet. It is called "Alien's Travel Permit" or "PSB permit".

How to get the travel permits?
Because we are a LOCAL travel agency, it's quite easy for you to get a travel permit with our help. Simply send us your
1) full name;
2) gender;
3) date of birth;
4) passport number;
5) nationality
- all exactly the same as on your passport. And you need to tell us your occupation because journalists and people that could be involved in political matters could be revoked (they need more complicated procedure to get a permit).

You need to apply for the travel permit at least 10 days prior to your entry date. Please note that if you do not book any tour from us, we can not help you get the permits. No travel agency can provide "permit-only" service. Here is a note from Lonely Planet:

"Lhasa PSB will not issue travel permits to individuals and will direct you to a travel agency. Agencies can arrange a travel permit to almost anywhere but only if you book a Land Cruiser, driver and a guide."

Important Issues about Tibet Travel Permit
1. All kinds of people can get Tibet permit through a travel agency except diplomats, journalists, and government officials who should travel to Tibet under the arrangement by the Foreign Affairs Office of Tibet Government.

2. After having a permit, a travel agency could buy you the air tickets, and you can pass the check-in at the airport and the check point reroute with the permit.

3. Only the guide could hold the permit after you establish yourself on Tibet land. You are not allowed to bring with it and travel to anywhere you want because except Lhasa you need another permit named as "Aliens' Permit". Otherwise if you are stopped by the police you will be sent out of Tibet or have trouble politically and economically. Furthermore, the travel agency which helped to get you the permit will also be in trouble.

Therefore seriously not any legitimate travel agency could sell you the Tibet permit if you do not book a tour with them. We don't think you would like to be in trouble due to a "sold" Tibet permit after you pay a lot of money for the tour.

4. There is a service charge (by the government) to get the Tibet permit. And it takes time, paper work, transportation fee and delivery fee for us to get a permit for you, so ATT will charge an extra fee for the permits.

Other documents
Other documents are the same as you travel to other places of China - you need hold a valid passport with valid visa (issued by Chinese Embassy).

If you are coming to Lhasa from Kathmandu, you'd better get China visa from Kathmandu because no matter you've already got the visa in your country or not, you must get a visa in Kathmandu, which is regulated in an official memo between China and Nepal.