It really depends on you. If you're not just interested in getting a kitten, getting an adult cat from the shelter would probably be the most practical choice. You can choose a cat with a personality that suits you and her medical needs would probably be taken care of already.
If you really want a kitten, I think you might be doing better by getting one of your coworker's kittens. I really dislike those irresponsible people too, and I don't like helping their bad decisions, but she won't stop letting her cats breed even if she doesn't find home for all the kittens. Those kittens will probably have harder lives than the shelter ones, since more people adopt backyard kittens on a whim then shelter ones, and I often see children take a kitten home just to have their parents kick it outside after a while.
Either way, you are saving a life and you shouldn't feel guilty whatever decision you make.
