[Guide] Finding Grain Textures
There is a nice little pack of film grain textures on Behance that is free to download and use. Film Composite is another source for film grain and they have an entire section of free grain images and videos.
Note: If you are making a filter for a client, you definitely should not do that. Find a free resource or pay for one, but don't blindly download.
Edit the Image
Some grain images you'll find are going to be too high resolution. Open up your favorite image editor (Photoshop, Affinity Photo, GIMP, etc) and create a new image. Go with something like 1440x2560, 1080x1920, or 720x1280. For a single, static grain image, 1440x2560 is fine, but if you want dynamic grain, I wouldn't go above 720x1280 because you'll start to run into file size limit issues. If you are going with a single image, just load in your image and export it as a JPG file.
