I ordered my set off Amazon for a total of about $30 for all of them (I got a couple other things, and used the ad on program for a couple of them so I ended up saving money) Here's a link to the mid sized ones.
The largest one makes a 4in pom pom, the smallest one makes about a 1/2in pom pom
I've tried a variety of yarns, and I've found that some are definitely easier to form once you make them into pom poms, but the makers are extremely easy to use.
I will never go back to the cardboard circle method again. These are so simple, and the pom poms come out perfect every time. They look a little different based on what yarn you use and how thickly you wrap the yarn, but I'm gonna say that they are a uniform size and shape.
I definitely reccomend purchasing these if you make pom poms for your crochet or knitting. Or even if you just want to make pom poms for other things. (My five year old son loves the pom poms and calls them his "earths". So, to him, they are planets.)
There's also several videos on Youtube that explain how to use them, because the instructions on the package are a littler weird. This one is my go-to