I too have had this challenge and found that there are many amazing resources here free once you have posted enough times - as well as I recommend the
eBook if you have not yet purchased that - the very same day I ordered the eBook I wrote out two proposals and both clients signed within two days and that was on a Sunday night! You are in the right place. As far as an introductory letter - I try to find a sample of what I am looking for or thinking of here - or google search it - even check out the samples in MS Office - I have used these and edit them to fit myself and my business. I would love to see what you come up with - and when I can figure out how to post them somewhere other than my site I will pop up some examples of what I use!
Patricia