If you missed the pre-sale then participate in their main tokens sale. That's the only way.

If you are talking about newly listed tokens then you can buy it right away on the first exchange (centralized or not) that will list them.

The way you asked that question, I think what's on your mind was there was a specific "best" crypto exchange that listing all newly tokens/coins. No it's not what you think. First listings (official) may come at different exchanges so your really need to head to that project's discussion platform to stay updated.