If you are looking some forums related to your niche, you can Google search it. For example, if your niche is tech, you can search for 'forum:tech' and you will get many sites that have forums for tech.
Similarly, you can also search for 'tech+forum sites list', and Google will display many sites that have such lists.
After visiting any of these sites, you can access
the forum posting sites list and register at any forum of your choice.