I'm sorry your parents are being such...a-holes. Dominick is right, they are not being very Christian.
There are some books that might give you some ammunition if you have to fight fire with fire. The ones I know of are geared toward homosexuality, but I think they might also cover transsexuality in a special section--or maybe some of the same passages apply.
There's Helminiak's What the Bible Really Says about Homosexuality. I started reading it but never finished.
Also, Hate Thy Neighbor, by Linda Patterson.
You can find one of these and use it to find other books on Amazon. Then see if the books are available at a local library or even a public university library. Even if you can't check out the books, you can read them in the library and take notes.
But in my secondhand experience, most people like your parents can't really be argued with, even when you use Scripture. You might need to just wait and see what happens. And if you can, maybe find a trans-friendly church in your area and talk to the minister/pastor and have some conversations with that person.
FWIW, I have a friend whose heavily Roman Catholic parents finally came around and are very supportive now. It can happen.