Themeforest is a huge platform with a lot of Wordpress Themes and Templates. It is part of the large Envato Market, which not only offers Themes for Worpress but also a lot more themes and resources for other content management systems. The number of themes and templates for Wordpress is continuely growing, in March 2020 - 48,607 WordPress Themes & Website Templates From $2.
They are searchable by categories like Corporate, Creative, Blogs, eCommerce, Retail, Entertainment, Technology and many others. If you purchase a license for a theme you usually get a package which consists of future updates and a limited period of support from the theme creators. Often there are also options to prolong the support period e.g. by another 12 months.
Customizations of your theme are normally not included in the support packages, but many themes come with a lot of customization options already included in the wordpress theme editing interface which should be enough
Themeforest sells some really popular themes on its platform you might have already heard from, for example “Avada”, “BeTheme” or “Flatsome”. To get yourself a good overview of the incredible amount of available themes on themeforest you should also take a look on the theme ratings and user reviews. The theme creators are able to also comment the reviews - if you see this it is a good sign that the creators are caring about their theme community.