Hi vjane89,
You can read more about foundations, box springs, or other support systems for your mattress and which type is suitable (and important) for different types of mattresses in post #1 here and the other posts it links to in the second paragraph.
While a suitable support surface (foundation, box spring, platform bed, etc) is important for a mattress … which one is best for your specific mattress will depend on the type of mattress you purchase.
If your box spring has any soft spots or dips in the support surface then it can certainly have a negative effect on your alignment when you sleep on the mattress and can affect the durability of the mattress as well.
If you don’t have any sleeping issues on the mattress/box spring you are using then you will probably be OK in the short term and I would probably wait until you buy your new mattress to buy a suitable foundation with it (or you could also use your mattress on the floor or put a piece of plywood over the box spring temporarily if you needed a more suitable support surface).
Phoenix