There's a whole host of manufacturers out there whose business is making double-glazing units - DGUs. Most 'double-glazing companys' not make the DGUs - they make the customised frames, then buy the DGUs to go into them from a supplier. You trouble is going to be that DGU manufacturers are just that - you are going to have to find someone willing to take on the job of fitting them. Perhaps a conventional glazing contractor?