Help please! Providing proof

Hello, 

I’m trying to find out if there is any specific documentation that I can have/will need to prove the diagnosis. For example; I am looking to visit numerous concerts, theme parks and so on, but am struggling with the crowds and spaces. I recently went to a concert with a standard ticket, but I started to panic half way through. Luckily, the staff were very understanding and allowed me to sit in the disabled area for the rest of the concert. 

To book tickets in the future, if I was looking to book disabled tickets instead of standard, I’m unsure of the proof I would need. I know someone who is 16, (I am 26 by the way) and has a card stating his diagnosis and that allows him to get the accessibility he requires. Where would I go for something like this if I can? 

Any help is greatly appreciated!