Hi
My husband is a martial arts instructor and has to be CRB checked. You have to be an approved body to apply for one yourself, and so in your case you would have to go via an "umbrella" agency who can apply for one on your behalf. A good one is Capita, look them up under Google. There are 2 types of disclosure you can get, standard and enhanced, its an enhanced disclosure you need for working with vulnerable groups and kids. I think his last one cost about £150, they invoice you afterwards, and they last 3 years.
Hope this helps, if you want to pm me if you have any other questions you are welcome.