A hands on cheap method:
Cut a disk of the right size out of a sponge. A thin sponge cloth as used in doing dishes will do, a thicker one will also work.
Put a thin layer of paint on a plate, put your sponge disk in the paint and then use it to mark your spots.
Try it on a spare piece of paper, to see how much paint you need and how much presure, both taking up paint and releasing it.
You will need to add to the paint in the plate as you use it.
For freehand work that is all you need. It will be more tidy if you mark out where to place the dots.
Use gloves and/or paint you can wash off you hands.
Back in school we used string glued to a bit of wood with a handle. If you need to do many spots you can also do that or glue the sponge to a backing and handle.