It depends on what you mean by non common altcoins. But going by your mentioned position of not in the top list of 100, I think you are talking of coins or tokens that are not yet known or not in the big exchanges.
Many projects that are not yet known have found their way in bancor to start with, so maybe I have answered bit of what you want. You can search the official website, you can check cmc to locate or trace the exchange official website.
https://coinmarketcap.com/