You need prescription. Go to your family doctor or walk in clinic.
If you're not comfortable dealing face to face with a doctor then get it online like Felix or UpGuys. You fill up the needed info, a doctor calls you and gives you prescription. Pay the fee and they mail it.
I used the Maple APP on iphone. It was a chat session with a doctor and got the prescription in a few mins. I had to ask and they included 3 renewals of the prescription as well.