If you're under 18, you really can't get on T without parental permission. Planned Parenthood requires you to be 18, because you must consent to treatment which you can't actually legally do until 18, your parents have to consent for you until then. If they tell you there is a way to do it without them, they are lying. If they're so sure, ask them to explain it to you, I bet they can't.
I know it isn't the answer you want to hear but it is the unfortunate truth. My parents wouldn't let me go on hormones because they never thought I was 'ready enough' so I had to wait until I turned 18- went to Planned Parenthood on my birthday in fact. I am not sure if the Planned Parenthoods in Idaho even do hormones, but there should be somewhere you can find doctors who operate with Informed Consent, which means no therapist letter required.
You should definitely try to get into contact with an LGBT center near you, they should be able to offer help and resources, and they hopefully will have a youth support group where you will definitely find people in the same situation.
