I bought the GTA premium online but don’t have the CESP
Solution 1:
Pause the game and go to the Online section of the menu. Click on Criminal Enterprise Starter Pack and it will "activate" the items. You can then go to each item's respective store (e.g. Ammunation for guns), and the items in the pack will be made free. If you sort the items by price, you should be able to find them more easily.