Unfortunately you can not find any Duel Links tournaments using google since they are all organized in private discord groups which is why I wrote this guide.
I will give you an overview over the biggest tournament servers. There also exist a lot of smaller tournaments which are usually advertised within the bigger servers.

The biggest Duel Links Tournament by far is the monthly event Meta Championship Series which gave out over 4000$ in prices last time:
Invite links get posted in the Duel Links Meta Discord server in the weeks before the tournament.
To join the Duel Links Meta Discord Server you need to be subscribed to https://www.twitch.tv/duellinksmeta MCS also has a 10$ entry fee Last time it was a 2 day event at 1pm EST / 7pm CET on Saturday and Sunday.
Duel Links Meta also hosts the weekly Single Elimination Event Meta Weekly with a 2$ entry fee. You also join by entering the Duel Links Meta Discord.

The secon biggest tournament is the Duel Links Pro Tournament:
To join this tournament go to this discord server (all other discords from now on are free to enter): https://discord.gg/uHKGVax
It has a 5$ entry fee and is held montly on Saturday 1pm EST / 7pm CET
They also host weekly free to enter Single Elimination Tournaments
They also have a very cool website where they publish a seasonal ranking of Pro Players based on objective tournament results. Check it out: http://stats.duellinkspro.com/

Next up is the Duel Links Challenger Series which is held on Sundays at 2pm CET / 8am EST / 10pm JST It ususally has no or only a low entry fee so check this out if you want to enter a free competitive tournament. To compete join this discord server: https://discord.gg/sQMMcDF for more information.
They also occasionally host the Challenger Cup which has an entry fee and much higher prices. The next one will be Saturday 17.03. 1pm EST / 7pm CET
It is organized on the same discord https://discord.gg/sQMMcDF

Duel Links Open is a weekly free to enter competitive tournament held on Mondays alternating between 1pm and 6 pm EST.
To participate join this discord: https://discord.gg/nX3Zsn2

Jayrabian Cup: A free to enter tournament organized by the popular Duel Links Streamer Jayrabian. Join this discord server to participate https://discord.gg/BGKcxVG

Lastly we have Team Wars. This is a team league where players join up to form a team of 12 players to compete against all the other teams to determine who is the best. Matches are played out over several weeks with the best two teams meeting in the grand Final. Each team pays a 50$ entry fee which goes towards the pricepool. First team will get 1000$ in Season two: Join here: https://discord.gg/HtD8yYb


Hope you will have as an amazing time playing in tournaments as I do :)