Here is a curated list of the best repositories currently available. These range from process engineering symbols to modern flat-style UI kits.
: While primarily a premium service, they offer a free download of 20 professional graphics to "try before you buy". These are vector-based and provided in multiple formats including .SVG, .AI, .EPS, and .PNG.
Use a tool like (free vector editor) to crop, re-color, and convert these files into a uniform color scheme (e.g., a dark theme for night shifts or a high-contrast theme for outdoor HMIs).
If you are building a custom interface from scratch or using open-source hardware, these libraries provide the actual graphical engine and UI components. LVGL — Light and Versatile Embedded UI Ecosystem