A polka dot is a dot found on a piece of fabric as a design. A polka dot typically sits with many other polka dots as part of the design. A spot is a round mark. A spot can describe what a polka dot looks like. A spot can also refer to a particular place. Finding "a good spot" means finding a good place. It's clever that the joke is using the word "spot" to mean "place" when talking about painting polka dots. 
Those who want to paint polka - dots have to find a good spot .