As you appear to recognise, you can't get a CRB check on yourself. (However anyone who will be working with children or vulnerable adults can now register with the Vetting and Barring Scheme).
The nearest thing to a CRB check, which an individual can obtain, is a copy of their 'police record'. Everyone has the right to receive a copy of all information about them which is held on police computers (except, for example, details of any ongoing confidential investigations). Such information will show all criminal convictions or, if appropriate, the absence of them.
Your wife can obtain a copy of her police record by submitting a formal application, together with a payment of £10 and proof of identity. Most police forces include details of exactly how to go about it (usually including a downloadable form) on their websites. To find the relevant information, google the following line (including the quotation marks but replacing xxxxx by the name of the relevant force, such as 'Suffolk' or 'Metropolitan'):
"subject access" "xxxxx police"
Chris