Angle
Great for device mockups. There is a free pack you can download, or you can purchase the full library for $79.
Artboard Studio
Go-to for brand assets. Has everything from apparel mockups to packaging. There is a limited free version, or it’s $12/month to unlock the full product.
Behance
Searching “mockups” in Behance will give you a bunch of options depending on what you need. Phones, glasses, boxes – you name it. There are a lot of free options out there, but you may have to pay a small fee for the better quality ones.
Creative Market
Like Behance, Creative Market has a lot of options when you search “mockups”. You’ll find a variety of mockups that you’ll probably have to pay for, but they look pretty real in the long run.
Figma
Figma’s Community section has a ton of resources. There are plugins you can download (Angle and Artboard Studio actually have plugins available through Figma) or there are files you can duplicate that you can use. Just search whatever item you’re looking for and have fun exploring!