All foreigners going to the Tibet Autonomous Region are required to be part of an organized tour that includes travel permits, a tour guide and a private vehicle with a driver (no private vehicle is required if you are just staying in Lhasa). There are no exceptions to these regulations. I have lived on the Tibetan Plateau for the past 9 years and previously owned my own travel agency. Though I now only arrange photography workshops, I still know which travel agencies to use and which ones to avoid.
If you are planning a trip to Tibet and want a recommendation on which agency to use, send me an email at firstname.lastname@example.org. I only recommend Tibetan owned and staffed travel agencies that have excellent safety records. Keep in mind that the cheapest Tibet tours are cheap for a reason! These extreme budget travel agencies, most of which are in Beijing and Chengdu, often have poor safety records, use guides that know little English and know little about Tibet, and have a poor customer service record. Many of these budget agencies do not care anything about your journey to Tibet, but rather just care about making some money off of you. For recommendations on a good, reputable travel agency to use while in Tibet, regardless of your budget, feel free to send me an email.
If you have any other questions about traveling in Tibet or need a recommendation on a travel agency to use, send me an email at email@example.com or stop by my Facebook page, which I keep updated with the current travel regulations.