Don't want to buy a costume? You can create one yourself.

[edit] Steps

Decide which Powerpuff Girl you want to be. Bubbles (sky blue), Blossom (pink), or Buttercup (light green).
Make, find, or somehow obtain a dress that is the color of the powerpuff girl you are being. You will want a knee-length, sleeveless dress that is a solid color, with a thick black stripe in the middle. If you cannot find with a black stripe you can find a belt or cloth to pin around you.
Get a wig, or style your hair like your chosen powerpuff girl. For Bubbles, have two short, round, blond pigtails. For Blossom, wear a big red bow with the rest of your hair in a long ponytail. Buttercup's hair may be hard to replicate, look at the link to see what it looks like.
Wear black shoes and white tights
If you want, you could use face paint around your eye to make it look like the Powerpuff Girls'. You could also use a mask.

[edit] Tips

This is a great costume idea for three people. If you have an extra person or two, however, don't despair! They can be another character (Mojo Jojo, Professor Utonium, Princess, Him, etc.).