In the years of constantly searching for free mockups for my pitching and presentations (I wish I could afford the paid ones but alas!) I've compiled my personal library of the best templates and links to those sources that always have a good option. You have to show the clients the realistic image of your future work. And creating a custom mockup for each project take too much time. Today, working on my pitches is easy and fast. I wish I had such a ready-to-use selection at the start of my freelancing days. So, here are some of the resources I use. I know how helpful it can be to have free templates at hand! I mostly work with books, magazines, and blogs, so this selection is the best for this sphere - Format selection. This resource offers more than 150 free creative and realistic templates I'm using daily. The second top resource I'm using regularly is Mockups-Design. There's a mix of free and premium templates, so you have to check the license first My last source is my own mini-library. Creating templates in Photoshop is not an easy task, but custom mockups offer a better chance to stand out. No need to make a unique design each time, create a couple of template to suit your line of work.
Actually creating mockups is not a hard task at all, it may become tedious it depends on the kind of the mockup you want to craft. But still, free mockups are the main go-to, it's just nice to save some time rather than creating them from scratch, like you've said just check the license!