I always take the lazy approach when possible. I forget where I found this file of balloons suitable for various manipulations.
IIRC, they don't upscale quite as well as I'd like, but 98% of the time, I just work with the original balloons in my stuff,
There's an example of it in use
here, in the lower left panel. In the original, Jill only had a one-word response with a tiny balloon, but I needed her to say more, so I pasted in a bigger one. It stands out a little more than I'd like, but works OK.
Edit: Hmm, it won't let me upload the file in JP2 or PSPImage format, which have the balloons as layers against a "blank" background....