Can you tell me where I can find good mockups that I can use directly in my browser? I would be grateful for your advice.
I think you should visit this site https://free-psd-templates.com/category/free-psd-flyer-templates/free-christmas-new-year-flyer-templates-psd/ where you can find a lot of layouts and apply them directly in your browser, which is very convenient because you do not need to download editing programs. I myself very often use them and am very pleased. You should definitely give them a try!
As far as I know, the different layouts can be found on freelance sites. Very often people just share their work with others who need it.