There are plenty of sites that sell triangle design patterns. Which are great for people that don't know how to generate the patterns using Photoshop or Affinity Photo.
trianglify.io is a site that allows you to generate you own tiangle patterns. These are great as desktop background or blog post background images.
Trianglify.io is a tool for generating low poly triangle patterns that can be used as wallpapers and website assets.
Once you find the pattern that you like it cost $4 to download. Which is pretty cheap for a graphic design. For a little more than a cup of coffee, you can have your own color and style.