If you Google editable mockups, you're more than likely going to find endless paid options. But if there's one thing I do with this newsletter, it's make your life easier (hopefully).
SmartMockups is only mockup tool I have ever used, and it offers both free and paid content.
This website allows you to create mockups in any category. I know I've mostly painted a "technology" vibe for you, but you can use this to work on social media posts, packaging, and even apparel. Ever wanted to launch your own clothing line? This might be your push to start now.
Note that when you sign up for SmartMockups (which, you don't really have to, but you get to save your work online instead of downloading which is cool), you get a 7-day free trial to their pro version (which... you don't really even to "opt-in" for. They just sort of hand it to you).