List of interesting sources (1 - hight quality; 2 - average quality; 3 - assets/others):

1.1. https://www.mockup.maison/
1.2. https://www.the-templates.com/
1.3. https://mockupsbrasileiros.com.br
1.4. https://layers.design
1.5. https://various-formats.com/
1.6. https://benditomockup.com/
1.7. https://liquidmockups.com
1.8. https://supply.family/
1.9. https://mockuperia.com
1.10. https://artdirected.design
1.11. https://www.youworkforthem.com/designer/1545/studio-a
1.12. https://creativemarket.com/makebelieve
1.13. https://creativemarket.com/M-up
1.15. https://smartobjects.co/product/unlimited-purchase/
1.16. https://formatmockups.gumroad.com
1.17. https://salumi.studio/store
1.18. https://mockupand.co/collections/shop
1.19. https://twodefine.com/Resources-1
1.20. https://creativemarket.com/Merakite
1.21. https://www.someother.studio/
1.22. https://supply.family/creator/alens-lidaks/
1.23. https://mmmockups.gumroad.com/
1.24. https://vibesmockup.com/

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2.
https://grafikstash.com/class/mockups/
https://marsmockup.com/
https://mockupoccasion.kr/
https://akzentmockup.com/mockups/
https://www.sedshop.co/
https://houseofmockups.com/
https://mockreality.shop/
https://fabricated.gumroad.com
https://www.semplice.com/supply
https://www.customscene.co
https://www.studiogruhl.com/kontextseries
https://creativemarket.com/users/HazardMockups
https://creativemarket.com/MrMockup
https://creativemarket.com/users/Eleven
https://tristanmillerdesign.gumroad.com/
https://stykkestudio.com
https://copalstudio.com
https://octobernovember.com
https://rebeccaberrington.com/shop/creative-assets/
https://www.wavehellostudio.com/
https://www.studiokynd.com/
https://harmonaisvisual.com
https://www.studiolines.shop
https://www.moyo-studio.com
https://creativemarket.com/the.goods.club
https://creativemarket.com/granastudio
https://blanks-templates.com/collections/all
https://elements.envato.com/user/helloDigi/graphic-templates
https://studio-standard.co/
https://tuomodesign.store
https://doronsupply.com
https://supply.family/creator/lukas-diemling/
https://www.onlyanother.co/study-materials
https://fockups.com
https://mokup.gumroad.com/
https://www.akoyastudio.com/
https://www.grabdrop.com.au/

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3.
https://www.youworkforthem.com/graphics/
https://martyr.shop/collections/all/glass
https://www.studio2am.co
https://bulbfish.design
https://tomchalky.com/product-category/illustrations/
https://www.design-syndrome.co
https://oneclickboutique.com/shop
https://creativemarket.com/Mad_Supply
https://www.s48mockups.com/
https://www.livesurface.com/surfacestore.php
https://shop.studioinnate.com
https://mockupline.com
https://creativemarket.com/printpixel
https://fm-artz.gumroad.com
https://creatoom.com/hi
http://bolderaja1.com/