According to regulation of Tibet Tourism Bureau, all international tourists are required to get a Tibet Travel Permit before travelling to Tibet. But a diplomat, journalist or a government official can not get Tibet permit via Tibet travel agency. Only the Tibet Foreign Affairs Office can handle his/ her Tibet Permit application.
If you hold a personal passport, have already have a China tourist visa, and just want to come to Tibet for traveling, it is highly unlikely that you have any problem to get a Tibet Travel Permit, unless TTB suspends Tibet Travel Permit application. But you also need to choose a reliable online travel agency. It will be better if this online travel agency has a Lhasa based tour operator.