Almost every youth club in my area holds a tournament, from U7s up to U18s. If you only do OA, use Full Time to find a local youth league, and then for each club in the league that might be of interest, Google their website and/or contact details. Often a polite email to secretary@<club-email-address> that you were wondering if they were needing referees for their tournament, and that you are a county registered level x referee will yield results.
Depends on what you mean by a tournament. If you mean a local one to you then it is usually just a case of keeping your ears to the ground and see if anything comes up. If you mean overseas tournaments have a look at Tournaments Abroad (http://www.refereeabroad.com/) and RefEx (https://www.refex.de/)