Where is UI/Default shader located?

I want to make my custom UI shaders by tweaking the default shader a bit. So,

Where can I find Unity built-in shaders?

My endgame is to make a shader that retains the Image's sprite and has customizable solid color (as solid as the pink color that we often see if there's an error in the shader). Also, the shader can be masked using Mask component, meaning the shader must have Stencil properties thingy.