I recently just applied for my graduate studies and I used my internship supervisor, a professor from my undergraduate studies and that was 2 years ago, an employee and a supervisor from a place that I volunteered at.

How soon do you have to turn in your application? If you have some time you can possibly volunteer for a short time maybe a month and afterwards you can ask them to become a reference for you. A rule of thumb is whoever you ask to become your reference you must ask them if they will give you a positive referral or not? There are people who will agree to give you a referral but it doesn't always mean they will be a positive one and instead they can actually be a bad one.

Good luck!!! If you don't get accepted this time then you can use the time to work on building your experience in the field so that when you write about yourself it can more relatable to the graduate studies. I didn't get accepted my first time but I didn't give up. Keep going and not let it stand in your way.