You are mixing things a bit, FOREX is about trading currencies, 500 index is different.
However, to simplify the answer, you need to find a company that trades it, in addition to all the international ones, as well as oil, gold, and even cryptos in some countries. Spreads are really low, 500 index, which is named US500 CASH on their platform, has a tiny spread of 0.70$ per lot.