I subscribed to SlideBazaar a few months back and I can say firsthand that the templates are superb. Their after sales support is super nice as well.
Slidesgo - An extensive catalog of free Google Slides themes and PowerPoint templates.
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TemplateMonster - TemplateMonster offers premium website and e-commerce templates.
Slides Carnival - Free quality presentation templates for Google Slides
SlideTeam - Transform rough sketches, notes into professional PowerPoint presentations.
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