I've never found anywhere that sells it individually, however. From what I've seen, they usually come in packs of 10-20, and those can cost anywhere from $20-$35. Do some shopping, see if you can find anything that's a better deal, or cheaper.
Use sticker applicators or printable sticker paper if you want the process to be quick and fairly stress-free. The result will look good from a distance, but up close it may not be quite as glamorous.
Use water slide paper for a better look and feel, but take more time and effort.
Use MSpaint for an average, pixelated look.
Use Fireworks MX, GIMP (free software), Photoshop, Paint.net, or any other image editing program that doesn't already come with your computer for better decal quality. Also for more detailed decals.