Attached Link: http://www.blambot.com/balloons.shtml

Don't know about plug-ins, but you should consider looking for a collection of free brushes or custom shapes for Photoshop. Especially custom shapes, which are easily resizeable because they're made up of paths. If you look for anyone offering custom shapes, where the shapes are comic balloons and the like, you should be able to download them, load them in Photoshop, and use them to your heart's content.

If you're unable to find any, you should be able to download some from the link I'm providing and either import them as paths to Photoshop to convert them to shapes or copy them from Illustrator and paste them into Photoshop as paths. So many ways to get to the same ending, really.