A list of every single mockup source I have ever used I think.
Mr. Mockup - Freebies
Bendito - Freebies from every collection
Mockup.Maison - Few Freebies
BLKMARKET - Freebies
Gumroad - Demos
Mockup World - FREE
Creatoom - Freebies
Freepik - Subscriptio Premium
Akoya Mockups - PAID
Bl4nco - only 2 freebies but wow the quality
Pixel Surplus - Free
Graphic Burger - Free - More than just mockups
Superunknown - Paid Music Mockups
Freedesignresources - FREE
Smartmockups - Free Online Mockup Generator