List your coin for free mostly mean that listed exchange have have low trading volumes or you want to increase their trading volumes and most of these exchanges are avoided by traders.
Listing your currency on a reputable exchange requires you to pay a lot of fees and then your currency is reviewed before listing.
easy solution is to include them in decentralized exchanges, or swaps if they are tokens.
hard solution is to develop it so that those reputable exchanges will list it for free.
i recommended Kucoin, Poloniex, and Huobi.