There really are no rules for spelling names. You can spell your name any way you want.
With the name Sadie, you can have the following options to choose from:
Sadie has several variant forms: Sada, Sadah, Sadelle, Saida, Saidee, Saidey, Saidie, Saydie, Sydell, Sydella and Sydelle.
Sadie \s(a)-die\ as a girl's name. Diminutive of Sarah (Hebrew) "princess", it was used as an independent name at the beginning of the 20th century. It is a very popular first name for women but an unusual last name.
Of interest, check out some of the most well-known household names that are extremely hard to spell accurately from HuffingtonPost.com.