As Gabby posted it is easy through your GP to get if it is something significant/traumatic that happened in your life and for short term use. However anti depressants for long term needs are not a one size fits all. What works for one may not for another and could require your GP referring you to a mental health specialist with knowledge in dispensing the more complex varieties.
You can go to a GP.... they may or may not prescribe one to you. They might actually refer you to a therapist so they can analize the situation and write something specific up for you.
Prescription-grade? Yes, but if you're just looking for a general mood-enhancer, you can get St. John's Wort over the counter because it's a vitamin.
Yes or a mental health clinic. If you've had something significant happen in your life recently it would be quite easy to get some.