The easiest method is employee groups on Facebook (and sometimes LinkedIn). You'll be able to comment on company pages, for example here: https://www.linkedin.com/company/starbucks - but you'll have a bigger response on Facebook, for example here: https://www.facebook.com/starbuckspartners
If you are required to do it on location, go to the business during their least crowded times and have a casual conversation with the employees. Make it feel friendly, and don't just start with questions - consider a compliment and small talk first. Ask your questions (So, I'm curious...) and record your answers afterwards. If you need lots of answers, or need them to fill out extensive surveys, ask the manager at these locations for permission - and tell them why it matters (for example: "I'm creating a startup that will do this really amazing thing, and I'm trying to figure out whether employees of Starbucks would use it"). Unless you are donating to the tip jar or charity, don't expect everyone to jump at the chance. If they express any irritation, simply thank them, apologize, leave, and go to the next location. In terms of survey platforms, SurveyMonkey, AYTM, or Google Consumer Surveys are ideal. The specific platform depends on what you are asking.
If you'd like to deep dive and create a plan to make this work right, pick a few times that work well for you: www.clarity.fm/ryandraving