I considered this too. Considering that a 'standard' dose can be way too much or way to little, depending on the person, you're not likely to have effective results without the care of a physician. There's also the health of your liver (and to a lesser extent, your heart) to worry about.
There are ways to get hormones without therapy (like an informed consent clinic), if you're dead set against seeing a therapist, but you should still be going through a doctor.
Part of getting a prescription is making sure that what's in that bottle is actually testosterone; not heroin or drain-o or wheat gluten. You never know, with those grey and black market drug providers. If you're going through a professional, at least you have some reassurance that you're getting what you're asking for.
Besides. You've waited since you were 7, what's a couple more months? A therapist isn't just going to tell you what you already know, but has helped other people with the social complications of transitioning (like having friends and family ditch them) and might be pretty useful to have around if/when those things happen to you.