Try to search your coin at Coinmarketcap then see the markets tabs. It lists an exchange that accepts the coin you search.
If the coin not registered in the coinmarketcap, you can't find it. Coinmarketcap is only for coin that is registered by the developers. Some coin are traded in exchange but not listed in coinmarketcap, when you see in coinmarketcap you can't get the coin.
You can do a forums search
https://bitcointalk.org/index.php?action=search for the most obscure things, occasionally the
Marketplace (Altcoins) would pop up, offering peer 2 peer trades.
The problem could be order sizes. Someone wanting to trade 200k of coins in one batch. At the sites listed on Coinmarketcap you can trade the smallest possible portions, recommended to never go all in or all out in between one day. Averaging down instead.