BubbleGen is tools that works inside Unity Editor and allows you to create bubble sprites (flat png images that looks like bubbles) that you can then use in your own game. Simply choose colors and patterns of your bubbles and export them as png files, up to 1024*1024 pixel size. BubbleGen makes sure no-one have same kind of bubbles in their project.
Package downloaded from Unity Asset Store will contain much more detailed instructions how to use BubbleGen.
Full editor view (yours may look different depending on your Unity Editor layout):
Resulting png files generated by BubbleGen (just infinitely tiny fraction of all possible):
Some example output images as created by BubbleGEN, in zip file: Download PNG example sets
Download from Unity Asset Store