I just purchased a few toppers to make a latex mattress, and I'm using my old S box spring and it seems perfectly fine. I was doing research on this as I wasn't sure what to do, and it seems some foam sites will naturally encourage you to buy their foundation, but I don't think it's necessary.
This is from FoamSweetFoam's website:
Decision #3: Foundation or No Foundation
Our mattresses do not need a foundation for support. If you need to put the mattress on something to achieve a desired height, it can be placed on your old box spring (if it doesn't sag), directly on slats (as long as they are less than 8 inches apart), or directly on plywood such as a Captain's Bed or Platform Bed. We offer a 9" wood foundation, but only want to sell it to you if you need one.
Bottom Line: If you need to raise in height the mattress you purchase from us, you can use anything that will give you support as long as it doesn't sag.