Learn how to make your own after the jump.
Lay out your pullover and put the paper or cardboard between the front and back (to keep the fabric paint from leaking through).
Dip the eraser of the pencil into the fabric paint and then dot off the excess onto the paper plate (too much paint will make your polka dots look smudged).
Use the ruler to measure how far apart you want your polka dots and then row by row create your dots (mine were about 2 in. apart).
Let dry and you're done. I only did polka dots on the front, but you could most certainly add them to the back or the sleeves.