I still feel the question of "sources for indecent dress by women" deserves a better treatment than we (including me) provided up to now. So let me try again.
- the key Torah verse prohibits erva in the Jewish camp
- the gemara will define a woman's erva as parts of her body, her hair and her voice
- most laws of tzniut are part of dat Yehudit and differ depending on time and place; all agree that some body parts are erva but the exact limits differ
A very relevant source not yet brought up here is a very complete and highly recommended article on tzniut by R Yehuda-Herzl Henkin in Tradition 37:3 [abbreviated RYHH below, page numbers are from the PDF linked and not the printed article].
First R Chaim Tabasky provides the overall framework
In Devarim 23:15 we read that Hashem walks in our (military)
camp, and that no matter of nakedness should be seen lest He leaves us.
From here we derive the prohibition of nakedness when in Hashem’s
presence, e.g., for prayer, Torah study, etc. The term camp, however,
implies that a constant restraint is required.
This verse in Devarim is the source verse given in nearly all discussions of tzniut because of its reference to erva
כִּי֩ יְהוָ֨ה אֱלֹהֶ֜יךָ מִתְהַלֵּ֣ךְ ׀ בְּקֶ֣רֶב מַחֲנֶ֗ךָ
לְהַצִּֽילְךָ֙ וְלָתֵ֤ת אֹיְבֶ֙יךָ֙ לְפָנֶ֔יךָ וְהָיָ֥ה מַחֲנֶ֖יךָ
קָד֑וֹשׁ וְלֹֽא־יִרְאֶ֤ה בְךָ֙ עֶרְוַ֣ת דָּבָ֔ר וְשָׁ֖ב מֵאַחֲרֶֽיךָ
"For the Lord your God walks in the midst of your camp, to deliver you, and to give up your enemies before you; therefore your camp shall be holy; that He sees no unseemly thing [erva] in you, and turn away from you"
Continues R Chaim Tabasky
The Rishonim consider the nature of nakedness [erva] and whether the
prohibition of uncovering certain parts of the body is fixed or
depends on social circumstances. All agree that certain areas of the
body fall under the Torah prohibition, while others may be drabanan,
or depend on custom.
The key gemara on tzniut is from Brakhot 24a which defines erva as a woman's shok (leg), voice and hair (see RYHH p. 1).
Rav Hisda said: The calf of a woman's leg is to be regarded as
nakedness; as it is said, "Uncover the leg, pass through the rivers"
(Is. xlvii. 2) and it continues, "Thy nakedness shall be uncovered,
yea, your shame shall be seen" (Is. xlvii. 3).
Samuel said: a woman's
voice is to be regarded as nakedness; as it is said, "For sweet is
your voice, and your countenance is comely" (Cant. ii. 14).
said: A woman's hair is to be regarded as nakedness; as it is said,
"Your hair is as a flock of goats" (ibid. iv. 1).
Rishonim will debate if the shok is above or below the knee, i.e., the thigh or calf. Rashi on Isaiah 47:2 mentions the arm (zroa) as erva and poskim will similarly debate the extent to which the arm should be covered (see RYHH p. 8).
The details of tzniut laws vary from time to time and community to community. The notion of differentiating between dat Moshe (Torah law) and dat Yehudit (custom) in the context of erva comes from Ktubot 72a-b according to many Rishonim (see RYHH p. 12).
(Mishna) What is dat yehudit? If she goes outside her home with her hair
(Gemara) Going out with her hair uncovered is a Biblical
prohibition for it is written "he shall uncover the head of the
woman". And a Tanna in the academy of R Yishmael taught "This is a
warning to Jewish daughters that they should not go out with their
Biblically it is sufficient to cover her hair with her
head-basket [which allows hair to show through] But in accordance to
dat yehudit it is prohibited for her even to go out with her head-based [rather a more thorough covering is required]
Indeed the Shulchan Aruch (OC 75:1, EH 73:1) doesn't detail the specific laws of tzniut but refers to local practices and local places. See also Mishna Brura 75:2.
Interestingly in some cases, tzniut customs have become less constraining in recent times, see e.g., Rambam in Hilkhot Ishut 13:11 where he notes women of his time used to go to the market with a veil covering their entire body!
R Michael Broyde wrote an entire monography to elucidate whether the obligation of covering a woman's hair was a Torah or rabbinic commandment and concludes
I have set out to investigate this topic in the footsteps of the great
decisors, and I tried to search all the books I could find to gather
the views of the Rishonim on hair covering for women, and I have
discovered that many of them — Tosafot, Rosh, the Tur, and Terumat
Ha-Deshen in particular — established the prohibition for a woman to
go with her head uncovered as a violation of dat yehudit and a
subjective rabbinic prohibition.