The inexpensive foam stamps found in the t-shirt section of Hobby Lobby make great imprints for mono printing. Spread the (Gelli Art or acrylic) plate with paint (tempura used here). Then press the plate with one or more stamps. It works best to dump the paint onto a clean page after each press on the plate. In other words, use the plate as a stamp pad. After each press of the plate "stamp" the paint off the foam stamp. That frees it to accept more paint with each stamp on the plate (otherwise it can't absorb more and it slips around and doesn't leave a crisp impression on the printing plate).
The benefit is a "two for one." As you make one mono print page you also make the bonus page from cleaning the stamps (and roller).