If you work in healthcare, you will have had a DBS at some point. However, when you locum, you need to take care of your own DBS rather than it being ordered by the company you intend to work for.
Locum Passport offer an Extended DBS service. Once you have your certificate, you may also want to apply for the Update Service that will allow you to keep your certificate up to date for £13 pounds per year(but could save you money in the long term, as employers do like to see recent(within one year) certificates or for locums to be on the update service).
NOTE: If you already have a standard or enhanced DBS certificate
You will need your certificate number to register for the Update Service. You must register for the Update Service within 30 days of the certificate being issued.