If you mean the trailer in the official channel, there isn't an exclusive Youtube channel for this because they're usually released by the the company who owns the rights of distribution, like Avex Pictures, Kadokawa, Bandai, Lantis, Pony Canyon and more. Since it's too much work to check individually each channel for the trailers I just use this site here
https://www.livechart.me/, they always update each new anime with its respective trailer.
