I looked at a couple of US retailers I know and have used and found the set in the Pacific Northwest: http://www.132slotcar.us/store/product_info.php?cPath=21_32&products_id=7306
It’s not unusual for the Scalextric store to be out of stock of sets that have been discontinued but for plenty of sets to still be available with retailers. Although the Platinum set is slowly disappearing, searching “Scalextric C1374” I found sets in stock around the world - UK, Australia, New Zealand, Singapore, France, Denmark, Germany, Sweden, Norway and Belgium.
If 132slotcar.us doesn’t work out, scroll through the list of North American retailers via the ”Stockists” link in the header of this page and contact them directly. Not all have an active web store. Good luck!