Mattel stopped making viewmaster projectors. Decades ago. Vintage viewmaster projectors are relatively cheap online and you can find one around $40 online. They are very well built for items that are over 40 years old so there are lots to choose from. If you don’t want to enjoy your reels without pressing a viewfinder to your face, vintage projectors are your only option (but it’s good because vintage has character).
What model should you buy?
Sawyer (which eventually turned to Gaf) is a solid brand with sturdy machines that held up for decades. They seem to be the standard/must pervasive brand. The main issue I’ve noticed is sourcing a new opaque bulb. A normal bulb may work fine but may not be a bright as the light isn’t being focused.
3D Projector
If you can get your hands on the Stereo-Matic 500 projector you can view your reel in 3D. You’ll need a set of polarized 3D glasses to see your image pop off the screen. The kicker is you need to project the image on a surface that works with polarized images well. A regular wall, screen, or cardboard won’t work. They will scatter the polarized light. A surface including a silver screen (just found it’s not a metaphor), a glass surface, or metal will reflect the light back properly and create the 3D effect into your glasses.
Make your own viewmaster reel
Since the Mattel personal reel machine’s return is still unknown for over a decade, DIY may be the way. The View-Master personal stereo camera let’s you take two side-by-side photos. It’s also discontinued so you may have to find a vintage one at Ebay or Etsy. Before sure to use positive film like Velvia 50 so you can cut the film directly and use them in a viewfinder. Viewmaster has (you guessed it) a vintage film cutter out there but a steady hand and an exacto knife will do. Check out this tutorial for a more depth tutorial to make your own and slide it into your vintage viewmaster projector (especially if you don’t want to drop $200+ for a highly specialized camera): https://www.instructables.com/DIY-Viewmaster-Reels