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.
SlideModel.com - Create attractive presentations with our professional PowerPoint templates, slide designs, and themes. Use our PPT templates and slide designs to give unique appearance to your presentation.
BaseTemplates - The easiest way to build your pitchdeck and raise money
Slidesgo - An extensive catalog of free Google Slides themes and PowerPoint templates.
Pitchdeck - Pitch Deck is a web-based solution to you create engaging presentations.
SlideTeam - Transform rough sketches, notes into professional PowerPoint presentations.
Basic Seed Deck - Very simple way to structure your seed deck