Hi Andy,
As you probably know, National Instruments does not manufacture or sell DIM power supplies however there are companies that do produce DIM power supplies that will do what you require. A company called Phoenix Contact (http://www.phoenixcontact.co.uk/) does sell DIM power supplies so it might be worth speaking to them to see if they have anything that meets your needs.
Sorry I can't be of any more direct assistance on the matter,
Ian C
Applications Engineer
National Instruments UK & Ireland