I hope you all find what you are looking for.
Just a thought though - if your birth parents truly don't want to be found, no amount of great letter writing will change that.
I'm an adoptee too, so I know where you all are coming from. Just know that you may end up dealing with some rejection. Especially if your birthparents have not shared their adoption experience with their current family, etc. Getting letters sent special courier may end up causing a problem between them and their family, and that may filter down to you.
Just my two cents.
Again I wish you all the best and hope you find what you need.