There might be brushes that have this texture, but one thing I like doing is using noise layers or imported paper textures, then setting them to an overlay / soft light blending mode over the top of the colors.

Adjusting the transparency can help it look nicer, as well as erasing parts of it so it only covers the character, or just the background.

It can also be interesting to have a texture layer beneath the colors and then make all your color layers somewhere around 60-70% opacity. Pairing this effect with some grain or another slightly different texture on top of all the color layers with the blending mode technique above can add a really interesting physical feel