This varies depending on the size of the coffee shop. If you are looking to hire a few baristas, ex-employees would be your best option.
Once you find somebody interested in the position, it's worth having an interview/job mentioning with them about why they think this is their perfect job, what their long-term goals are, and how many hours they are able to work per day or week. It could also be beneficial for both parties if one of the employees has some experience working behind a barista stand previously. Knowing that some people can handle leading while others thrive as followers will help determine who should hold which role within the group and ultimately allow for more productivity and better quality espresso drinks!