The best way to get someone's number is to get to know them and get them to offer it through their own choice. If it is a matter of urgent concern, perhaps you could find out who his managing company are and get in touch with them so they can get in touch with him.
If you have a mutual friend, perhaps you can get the number from them for his cell phone. This will give you a contact to him and make it easy to explain where you got his number when you do.
You may also be able to contact Adam Lambert's agent or try and contact him via a social media site such as Twitter or Gumtree. This will allow him to contact you back if he feels the need to.
If you do receive a number that is claimed to be Adam Lambert's phone number from a stranger, it is best to stay on the side of caution as you can never know exactly who it is until you meet them and it may not be Adam Lambert you are meeting. To avoid coming to any harm, if you do get his supposed number never trust it.
So, there is no way to get Adam Lambert's personal phone number except if you get it from him. Always beware if you do get his number and you certainly won't find it in the public domain.