you wont find a dental nurse sample on goggle, what you'll find is just samples of letter references that are neutral, but you cant really use them because they don't really apply to dentistry, maybe they would give give an idea.
What I did, actually one dentist even suggested it to me, that I should write it out. since they don't have the time or the "words" to write one out, I wrote my own reference letter.and of course he view and read it too, before he signed it
Don't know how ethical that sounds, but the purpose of reference letters are just to show your strengths and why they recommend you.