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.
Zentation - Affordable webinar software. Create online presentations with synchronized video and PowerPoint.
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.
authorSTREAM - authorSTREAM enables presenters to upload & share PowerPoint, Keynote & PDF presentations online; embed in blogs, convert to video and share on social networks.
Slidesgo - An extensive catalog of free Google Slides themes and PowerPoint templates.
VCASMO - VCASMO is a multimedia presentation solution for photo and video slide shows, business presentations, training, academic teaching, sales pitches, and seminars.
SlideTeam - Transform rough sketches, notes into professional PowerPoint presentations.