What is the ruling on selling spare parts available in the shop along with others that will be provided later, and does the ruling differ if the customer knows that the goods will be supplied from another place or not, and is it permissible to state their price before they are supplied?

This is not considered selling what you do not possess if you do not finalize the sale until after the goods are available. It would be selling what you do not possess if the sale contract is concluded before you acquire ownership of them. It is not a condition that the customer knows you will procure the goods for him from another source, but you are not permitted to conclude the sale with him before acquiring them; because then you would have sold what you do not own.