How about "Thank Heaven for Volunteers"? or "There's a Special Place in Heaven for Volunteers",or "Volunteers are Shining Stars"?

Volunteers are superstars, Volunteers are shooting stars, Volunteers light up the night sky, Wish upon a shooting/falling star~volunteers make your dreams come true.

Here's another idea for the star them.

Though we could search both near and far, you always be our brightest star!

We thank you so much for all that you are, you are invaluable to us, you're a shining star!

Hope this helps!

Juli